I watched Thermae Romae on the plane, and I certainly wouldn't spend money on it.
The reasonably ridiculous premise is that an ancient Roman architect (and bath connoisseur) finds himself periodically transported to modern-day Japan, via a time-space vortex that seemingly appears anytime he touches the bottom of a body of water. There, he find a culture that takes bathing just as seriously as does his own, and soon starts to develop a friendship with a young girl he meets over several chance encounters.
To its credit, the film takes the well-trodden gag routine of “man from the past enthralled/confounded by modern technology” and puts a fresh spin on it with the bath focus, a place you wouldn't generally think to go. It's at its best during the mildly amusing comedy moments of protagonist Lucius getting to grips with the many conveniences the Japanese have contrived, ideas which he then takes back with him and introduces to his home. This catches the attention of Emperor Hadrian and he quickly finds himself a well-respected celebrity.
Sadly, despite the absurd premise Thermae Romae somehow can't manage to create anything truly interesting. Some of the little things are genuinely comedic, like Lucius taking a bathrobe and trying to use it as a toga, his frequent nudity, and his tendency to refer to Japanese as “the Flat-Face People” and continually remark on their supposed status as slaves. However, these bits of charm are only found piecemeal throughout, a handful of bright spots accenting a general feeling of going through the motions.
More tragic still, its two stars, both of whom I actually quite like, are completely wasted. Abe Hiroshi spends a lot of time wandering around and being surprised, but is never quite given a chance to showcase all the quirks and details that make him such an entertaining character actor. He's also so aggressively Japanese it's difficult to believe him as a Roman. For her part, Ueto Aya could easily have been replaced by basically anyone at all without much difference.
In the third act Thermae Romae seems to realise that it needs to develop a plot in order to actually be a movie, so it goes on a jarring extended sequence where Ueto's character travels back to Ancient Rome in order to help Lucius do...something that involves fighting, I think, and whose neglect will change the course of history. Actually, what? She's worried that if one character, who dislikes Hadrian, becomes emperor instead of another one, who does, Hadrian will not be exalted in later times. How does that change history? We would have ended up with Nero's Wall or what? It kind of doesn't matter, as the whole thing really falls apart at this point.
Thermae Romae has a lot of potential but quietly fails to live up to it. It's not even bad, it's just kind of dull. It's kind of a bummer that it was the only Japanese film available on the plane, and if I'd been able to pick something else I'd have gone with that one instead.