Japan Sinks is really a 1973 novel, which got a movie that was released the same year and a TV series that was released the year after. This 2006 film is another adaptation. Netflix have done two series adaptations (a 2020 anime and a 2021 live-action drama) and there's a parody version called The Whole World Sinks Except Japan.
The internet gets weird with the English-language titles, incidentally, calling different versions "Japan Sinks", "The Sinking of Japan", "Submersion of Japan", etc. It's all bullshit. They're the same in the original.
You've already guessed the concept. It's a disaster movie. Japan is the world's number one earthquake zone, resting on a destructive triple junction of tectonic plates. Japan Sinks shows us what might happen if this went very very pear-shaped. I'm not the biggest fan of disaster movies, to be honest, but I thought this one was quite good. The special effects (five years before the Fukushima nuclear disaster) are terrifying... but they're not actually the film's main focus. I'd been expecting more running and screaming. Instead, we have quite a lot of discussions, preparation and gallows good humour. At the start, everyone thinks the country has 30-50 years. This gets revised downwards sharply, but our heroes still have a year or so. That might sound like a lot of warning, but what would you do if you were the Japanese Prime Minister? If you want to evacuate 125 million people in that time, that's a million people every three days. Can you get that many boats and planes? Should you start negotiations to buy Alaska or something?
Also, of course, Japan's politicians are a bit rubbish, even by the standards of politicians. Tomoko thought the film flattered them, even though these are its two Prime Ministers:
(a) a noble chap with a big heart who's thoughtful, nice, etc... but he also wonders if the right thing to do might be just not to evacuate anyone and let the whole country die.
(b) a cold bastard who's going to lie to the public and say they've got five years instead of one, played by the always-excellent Jun Kunimura (who's the best actor in the film).
The film doesn't seem very well regarded in Japan, at least compared with the original. There are some scathing online reviews. Tomoko had heard that they'd shoehorned in a romantic subplot, which didn't impress her because she just wanted to see Japan sink. She also didn't like Kou Shibasaki's enunciation in one of the film's key roles.
In fairness, our comments to each other had turned a few scenes into comedy.
1. A small girl goes to see her mother, who's woken up from a coma and manages to utter a few words before dramatically dying. My suggested explanation of this coincidence was that the mother died because her oxygen mask had been removed. By her daughter. On screen. So that they could exchange a few photogenic words. (Laughing at this scene unfortunately makes the tragedy less emotional.)
2. Motorcyclists should wear helmets. Also, our three heroes are all on that one bike at once, with one of them being a child.
3. Then, later, Male Romantic Lead (Tsuyoshi Kusanagi) goes to see Female Romantic Lead (Kou Shibasaki) for one last meeting. She's a member of the Hyper-Rescue Team in the middle of a national disaster. How did Kusanagi get there so easily? (I don't think the buses are running.) Why are Shibasaki's clothes and hair so clean? Why do they get that enormous tent to themselves and, as a member of the emergency disaster rescue team, doesn't Shibasaki have more urgent things to be doing right now? When she suggests they have sex, do they really have time for that? (Maybe if it's a five-minute quickie.)
4. I was pulled out of the big goodbye scene by that song on the soundtrack. No, not incidental music. A song. A duet, no less. It's like watching a music video.
5. Does the science hold up? What about the nuclear power stations, for instance? (Theoretically, this also applies to the original novel, since Japan's first nuclear power station went online in 1963, but it's a more obvious question in a 21st century film.) Might all those final explosions have triggered another tsunami? Also, obviously, the reconstruction afterwards will be a nightmare. (If this really happened, you'd see a massive international aid effort... but this film has a slightly unsettling nationalism in which the rest of the world cuts Japan loose and people often choose to die rather than live abroad.)
Tomoko found the romance thin. I quite liked it, although it's more effective in the earlier scenes, when the film's not imitating Armageddon. The self-sacrifice, though, is pretty good and made Natsuki cry.
It's okay. It's still a disaster movie, eventually, but most of it is a "hasn't happened yet" with little conversations and sometimes-interesting glimpses into people. I liked the scene where Etsushi Toyokawa tells the Japanese cabinet exactly what they're facing and loses it with the inevitable idiot who thinks any news he doesn't like must be a lie. The destruction is scary. I'll be watching some other Japan Sinks adaptations for comparison and I'm quite looking forward to them.