The Door into Summer (2021)

The Door into Summer 2021 movie - Pito the Cat (Petronius the Arbiter)

Having been treated to an uncharacteristic flurry of snow during a February cold snap in London, made worse by the sedentary lifestyle imposed by the coronavirus lockdown, I was looking forward to opening the door into spring. Instead, I chanced upon the trailer for The Door into Summer, a 2020 Japanese film adaptation of the classic 1957 novel of the same name by Robert Heinlein. It’s a science fiction story about love, loss and a lengthier cold snap in cryonic preservation to escape the pain.

The full title of the film, The Door into Summer – To the Future with You (夏への扉  – キミのいる未来へ), sees the addition of a tagline alluding to the story’s enduring relationship. Or rather its two relationships — one feline, one female — and over the decades the latter has been met with one or two raised eyebrows.

The film’s trailers imply some differences to Heinlein’s original story, but the underlying theme is the same. It’s about cryonic preservation as a freezing fast track to the future, coupled with more temperate time travel to the past to solve the problems that keep two people (and a cat) apart in that future.

The film is directed by Takahiro Miki who also directed My Tomorrow, Your Yesterday, a 2016 film in which an age difference in a relationship plays a major role (as it does in Heinlein’s story) although presented with a twist. And Heinlein’s 1957 story is updated for today’s audience by writer Tomoe Kanno, whose first foray into time travel was writing the screenplay for the 2010 film The Time Traveller: The Girl Who Leapt Through Time.

Heinlein’s story moves back and forth between 1970 and 2000. Written in 1957, everything he wrote in the book’s present and future was therefore technologically advanced compared to the time in which he was writing it — it’s one of the key elements that made (and still makes) many of his stories so enjoyable to read. Today, readers continue to relish recognising the inventions he wrote about in the 1950s that are now in everyday use — and The Door into Summer is no exception.

In the film adaptation, the story moves back and forth between 1995 and 2025 — and this time setting might be for two reasons. Firstly, 1995 was just after the start of everyday use of the world wide web (for the general public, anyway) and largely pre-mobile phones (even in Japan) and certainly pre-Google (which didn’t arrive until 1998), which means the technology and setting of a film set in 1995 can be made to feel satisfyingly retro. Also, the audience for this film will probably be in the 16 to 30 age range and unfamiliar with life in 1995 — having been born between 1991 and 2004, the gadgets of the day (or lack of them) will be quaint enough.

The Door Into Summer

The film’s past in 1995 will therefore provide enough contrast to its future in 2025, a future which no doubt will be ramped up in terms of technological innovations compared to what is likely to be rolled out in the four years between now and 2025. We know from the trailers it won’t be flying cars (maybe driverless ones) but we do know that humanoid robotics, revival from cryonics and other forms of time travel will all be successful in the film’s timeline, albeit some still rather top secret.

I hope, really hope, the film doesn’t disappoint the older Heinlein fans who are often miffed that more of his stories haven’t be adapted (or successfully adapted) for screen. There have been about half a dozen other film adaptions of Heinlein’s novels, probably the two most well-known of these being The Puppet Masters (1994), which was a faithful enough adaption starring the always reliable Donald Sutherland — and Starship Troopers (1997), a film which was, unfortunately, an insult both to Heinlein’s genius and the audience’s intelligence. You can find a full list of the novels and screen adaptions here.

However, the popularity in Japan of the leading actor in the 2021 adaption of The Door into Summer should more than satisfy the younger audience in Japan, whether or not they have ever heard of Robert Heinlein.

The original Japanese release date of 19 February 2021 was postponed due to the coronavirus pandemic. Now due for release on 25 June 2021 in Japan, it will be distributed by Toho and Aniplex.

There are three trailers and/or teasers on the film’s website, which may or may not be viewable in your region. If not, there are two versions on YouTube: a long version on the Japanese Cinema Today channel without English subtitles, and a short version on the US Panap Media channel with English subtitles. All commentary in this article is drawn from the trailers, teasers and summaries on the film’s website — along with a thorough knowledge of the book.

All film images that appear in this article are from the film’s website.


In writing The Door Into Summer, Heinlein was, as always, sharing his vision of the possibilities for the future that he, as a citizen of the 1950s, thought we might have achieved by the year 2000.

As well as artificial intelligence, the idea of voice assistants, videophones, eReaders and electronic payment systems are part of the protagonist’s future in the year 2000. Self-driving cars are already in use by 1970.

The Door into Summer 2021 movie

The protagonist in the book is a robotics engineer named Daniel B. Davis who invents AutoCAD and the Roomba, albeit by different names, and is working on developing a flexible device which would carry out all other housekeeping skills inside and outside the house, preferring to use the term automaton over robot because it won’t be in human form, at least not in the book (they are in the film).

Ideas presented in the novel which are yet to make it to us today include electrostatic velcro, self-cleaning clothes, and how to regrow teeth (titanium implants and gum grafts are about as close as we’ve got in 2021 with that last predicament). Interestingly, Heinlein does predict the 1987 stock market crash.

As a robotics engineer, the protagonist in the book relies heavily on being able to code. Although a language is not mentioned by name in this novel, Heinlein was fond of FORTRAN in some of his stories — it’s the oldest official programming language and was made commercially available by IBM the year before Heinlein wrote The Door into Summer. Or maybe he was thinking of a beta version of LISP which was released the same year he wrote the novel. Today, LISP forms part of ROS (Robot Operating System) with Python and C++, a subset of which (I predict) could be referenced in the film. It will be interesting to see which it is. For a nice overview of robotics programming languages, head over to RobotIQ.

The coding referred to in the book is for the feedback control systems and Thorsen memory tubes which form the basis of the artificial intelligence utilised by Heinlein’s robots. These concepts had been introduced by Heinlein in his 1941 novel Waldo and artificial intelligence and neural networks were revisited with the neuristor in his 1966 novel The Moon is a Harsh Mistress.

But the main technological thread in The Door into Summer is cryonics — mostly referred to in the novel as cold sleep or suspended animation — where people can enter a contract for their bodies to be reduced to a cold enough state in order to take a one-way trip to the future, mostly it seems to escape something unpalatable in their past — and much to the annoyance of those living in their future who view the constant stream of unsophisticated country bumpkins coming from the past as spoiling their shiny new technologically advanced world.

Although the practice of cryonics is carried out in real life today, it is only performed after clinical death, unlike in Heinlein’s story. Today, in 2021, no mammal has ever been reported to have been revived from cryopreservation and the procedure is regarded with scepticism and is still a pseudoscience. You can read about the procedure on cryonics.org, the website of the Cryonics Institute, an organisation that carries it out.

As for Heinlein’s vision of gadgets that were good enough not to become defunct even in the face of advanced technology, the humble slide rule (or slip stick) was enduring throughout his books, to the point of overlooking the idea of the pocket electronic calculator. I suppose that must be chalked up to an anomaly that managed to slip through time.


Heinlein’s novels delight and infuriate today, as much as they did back in first edition, and The Door into Summer is no exception. This is especially true when it comes to the story’s secondary relationship, a friendship bond between its older male and younger female characters separated in age by 19 years. But it’s not the number that defines the age gap that gets people talking when it comes to this novel — it’s that he is 30 and she is 11.

Some reviewers have read more into this than is there. But the text is clear that they are just family friends. They do, however, meet up in the future when they are closer in age via a combination of one trip back in time and three bouts of cryonics into the future.

Part of the problem with the misinterpretation of the relationship lies with the cover artwork of one printing of the book (1970 edition, 1974 printing, cover artwork by Patrick Woodroff) that features a scantily-clad female (an overstatement) that may have been misinterpreted.

The girl was later replaced by a robot in the 1970 edition’s 1977 printing, with cover artwork by Gino D’Achille that immediately brings to mind the words “Open the pod bay doors, Hal“.

The 1970 Pan Science Fiction cover may represent him leaving behind the relationship with his despicable femme fatale fiancé. Or it could be an amalgamation of three women in the novel: the fiancé in his past, the woman in his engineered future, and a minor character whom he meets somewhere along the way on his brief return to his past when he accidentally lands in a nudist camp. Yes, really.

Befuddling cover artwork and scantily clad women is relatively common with Heinlein titles, as well as those of other sci-fi authors of that era. I sometimes wonder whether the artists ever read the stories, or just the back covers.


In Heinlein’s story of The Door into Summer, the 30-year old robotics engineer protagonist is friends with Ricky Heinicke, his business partner’s 11-year old stepdaughter. She is the only living being that the protagonist respects and trusts — except for his cat, Pete, aka Petronius the Arbiter to give the moggy his full Heinlein name.

The cat is the most important character in the story and shape’s the protagonist’s life. His fiancé is hostile to his cat, which is one of the reasons he realises he hates his fiancé. Ricky hates his fiancé too, but that’s because she has a childish crush on Ricky and she adores the cat too.

Pito the Cat (Petronius the Arbiter)

Our protagonist — full name Daniel Boone Davis (known as Dan) — is considering cryonic preservation to get away from his abysmal life that, for reasons I’ll get to later, has been well and truly destroyed by his fiancé.

But what, he wonders, if he were to meet Ricky in his future — 30 years in the future maybe, when she will be 41 and he will still be 30? But such a gracious human being will surely be long married, he laments. Unless, of course, he can go back again to his past and change their respective futures…

In his future, Dan tracks down an irascible scientist who had, so it is said, invented a method of time travel — or temporal displacement — that a few years ago may have enabled another scientist named Leonard Vincent to travel 500 years back in time… Or maybe it was forward. It’s impossible to determine which way it will go. That’s one problem with the technique. The other problem is that it’s classified.

But Dan does manage to be temporarily temporally displaced back in time and at great risk of what lies ahead in his past, since his requested period of displacement is three orders of magnitude more precise than the fractious scientist is able to correctly calibrate.

But successfully arriving thirty one years and three weeks into the past, Dan is able to track down Ricky and suggest that she might want to travel into his future — which he assures her is far better than her present. Dan suggests that in 1980, when Ricky reaches the age of 21, she puts herself into the same cold preservation that he took, to be revived 20 years later in the year 2000, a few days after the date that he is woken from his own cold storage entered into 30 years prior in 1970. All clear so far? That way, Ricky will be 21 and Dan will be 30 and they can look forward to a future together, with Pete the cat, of course.

It’s totally up to Ricky whether she chooses to go ahead with this, because Dan will have been gone for ten years by the time she makes the choice. And in 1980 she does choose the future, but only after her only living relative has passed away.


The Door into Summer 2021 movie

But the events in the screenplay aren’t quite the same as all that.

In the film adaptation, the protagonist is renamed Siochiro Takaura (played by Kento Yamazaki). He remains a robotics engineer in the updated story, but additionally he has perfected the plasma storage battery. Another change is that Siochiro has a humanoid robot accomplice (played by Naohito Fujiki).

The love interest Ricky is renamed Riko (played by Kiyoharo Hate) and is the daughter of Siochiro’s business partner Kazuto Matsushita (played by Hidekazu Mashima). Riko is, thankfully, a bit older in this story (16 years old, rather than an eyebrow-raising 11).

In the book, Dan’s fiancé is Belle Darkin, who is also his secretary and bookkeeper — and, as it turns out, a polygamous fraudster and convicted felon. In the film, the fiancé is renamed Shiraishi Suzu (played by the mononymous Natsuna). As to the nature of her criminal antics in the film, I don’t yet know, but I suspect they’re no better than in the book.

And then there’s Pete, now known as Pito in the film, a ginger ale-lapping cat who has an influential role in the story — and on the title — appearing mostly at the beginning and at the end of the tale. There is no cat in my household but, from the events of my own childhood, I purrrrfectly comprehend Heinlein’s description in the book of the inordinate amount of time that has been consumed opening doors for cats — since the dawn of civilisation, the protagonist calculates it to be 978 man centuries…

In the book, after his malevolent and now ex-fiancé Belle and ex-business partner Miles Gentry jointly defraud Dan out of his company and inventions, it kick-starts his desire to be put into cold suspended animation for a long enough time for both of the fraudsters to be dead and buried by the time he is revived.

But Dan decides on 30 years — which is long enough to annoy his ex-fiancé that he is still young when she is old. Having learned, so harshly, of the venom that runs in her veins, he is sure that age will rile her the most. (I should just clarify here, I’m not saying that Belle will be old at 57, which is the age that she will be when Dan is revived from his three decades of hibernation — it’s Heinlein who’s saying she will be old.)

Unfortunately, Dan’s cryonic hibernation is involuntarily brought forward when Belle uses it as a way of silencing him about the fraud she is in the process of committing to steal away all his inventions and patents. As a result of her plying him with a cocktail of cataplexic and hypnotic drugs, Dan is forced into the cold sleep facility and wakes in the year 2000 — without his beloved cat — and without being able to say goodbye to Ricky.

The Door into Summer 2021 movie - cryopreservation and the long cold sleep.

Being without Pete the cat is what Dan finds most distressing since he had never planned to be living in the future without him. But he does eventually meet up with Pete — and Ricky — in the past and they both get to join him in his future.

Dan also satisfyingly learns of the less successful fates of his two fraudsters and his hoped-for revenge on his ex-fiancé is pleasingly realised when he meets her “jumbled ruin” in his future. We infer from the text that Belle is now more Blanche DuBois than Wanda Gershwitz.


In the book, all of Dan’s brain functions, thoughts and memories appear to be unaffected from the moment he was frozen in 1970. When he awakes, he recalls having been under the influence of the zombie drugs that Belle plied him with, speculating that cryopreservation must have cleansed his body, sleeping away the effects in the intervening 30 years. He argues the same for having kicked his unhealthy nicotine habit.

Personally, I didn’t buy any of that, because the so-called cold sleep is supposed to stop all physical processes and we might presume that the drugs would be as potent as they were when Dan went under. I thought it when I first read the book years ago, and I thought it when I read it again recently.

But then I rethought it in the light of the ultra-low temperatures being bandied around in the press in 2021 for the length of time and limited storage life of, for example, the Pfizer coronavirus vaccine, not to mention the fact that freezing drugs may change the molecular structure of medication.

So that’s alright then. Maybe the potency of the drugs that Dan was plied with would indeed wear off. For example, the Pfizer coronavirus vaccine is stable for only 6 months at -70°C, and for just 5 days at the 2°C to 8°C temperatures of a typical medical refrigerator. I then realised that, unlike cryopreservation in ‘real life’ which is done at extreme temperatures of around -196°C, Heinlein’s cold sleep is more of a hypothermia hibernation, achieved at just 4°C, which he reminds us is when water reaches its maximum density with no ice crystals (or, as every secondary school physics pupil recalls it, the temperature limit below which we see ‘the anomalous expansion of water’). I then jumped back to reality realising that it’s just a science fiction story, and moved on.

Dan does, however, start to wonder whether his brain has been affected after all and whether he is suffering from amnesia after he sees his name on two 1970 patents for robots that he doesn’t remember inventing in his old life. It’s the work of that duplicitous ex-fiancé again. So Dan goes back to his past to prevent Belle from doing her dastardly deeds.


How this science fiction story written in 1957 and set in 1970 and 2000, translates into a film made in 2020 and set in 1995 and 2025, is yet to be seen, but the trailers imply that the events which unfold are not quite the same…

As well as Riko being older, the trailer implies that when Soichiro wakes up in 2025, he finds out that Riko died soon after he left in 1995, therefore he tries to change his past in order to change their futures.

In the scientist's lab where the time travel takes place

In the book, Dan is working on revolutionising the field of robotics to free women from the repetitious and unnecessary drudgery of housework with robots taking on most household chores. Remember, this was written in the 1950s where the role of the woman at home was an unfortunate, but tolerated, aspect of society — and to some extent, in some societies, it sadly remains that way today.

That aspect would never work in a film both made and set in the 21st century. Or would it? Here’s a devoted fan’s proclamation on the leading actor’s Instagram account in 2021: “Please end my misery and marry me, I can clean and will learn to cook.


I, too, am rather excited…not so much about the cast, about whom I know very little, but the prospect of seeing this time-jumping, blindsiding, freeze-thawing Heinlein classic adapted for the screen. I just hope there are subtitles.

In advance of the film’s arrival, you can listen for free to the whole of Heinlein’s novel on the US Baudio channel on YouTube. It takes just under seven hours and is splendidly narrated by Patrick Lawlor, expertly depicting the book’s dozen or so distinctive characters.

There are a few editions of the paperback on Amazon and plenty of well-thumbed copies on eBay and Abe Books. As I said, they come with an array of cover artwork that often doesn’t convey anything relevant about the story and often feature the female form in a costume that leaves nothing to the imagination (or in the case of the 1970 cover, no costume at all).


As well as the protagonist, Daniel Boone Davis, being based, so the story goes, on Heinlein’s own engineering attributes, Pete is said to have been based on Heinlein’s own cat, and the title of the novel on his wife’s observation of what their cat was always looking for.

This certainly shows how unexpectedly an idea for a story can emerge. What’s not so easy is being able to write a whole story in 13 days, so the story about this story goes.

I, too, have an idea for a story influenced by events in my life, but that particular roman à clef is in its own suspended animation for the future. (As Robert Heinlein said: “Don’t tell the reader about the background; let him gather it from what happens.”)

The Door into Summer will be released in Japan on 25 June 2021 (worldwide release dates still to be announced).

Related article: Time Enough for Writing a Sonnet.


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