It's one of Akira Kurosawa's post-war movies, which I regard as essential to know when addressing the film's themes. However in addition, it's also reminiscent of Frank Capra.
Our heroes are Chieko Nakakita and Isao Numasaki and they're on a date. It's Sunday. That's the film. No samurai, no death, no courtroom dramas. They go to the zoo, try to see a concert, get caught in the rain and do other date-like things. It's straightforward, occasionally whimsical and would have made for a lovely James Stewart movie, although the version Kurosawa filmed is more brutal in how it's exploring its heroes' poverty.
You see, Japan in 1947 was a tough place to be. The country had been all but destroyed by its own war of aggression, as a result of which it was now hated like poison by its neighbours. It had been nuked. The old government had been kicked out, Americans were running the place and now they were attempting reconstruction. Kurosawa's two heroes here have almost nothing, scraping together their date on so little money that they'll be in dire straits if a cafe overcharges them for two cups of tea. You'd expect them to be scrounging discarded items on the ground or selling their coats on the black market... but they don't. They maintain their standards. Numasaki refuses dodgy money. Nakakita is a good girl, if you know what I mean. There's corruption all around, with yakuza and black-marketeers, but our heroes refuse to associate with it. They're going to fight through this in a true Japanese spirit, or die trying.
That doesn't mean it's easy for them, though. Numasaki is depressed, sometimes to an almost scary extent. He'll shoot down Nakakita for being happy and can't see anything but darkness in their future. Nakakita on the other hand is little, chubby-faced and will work like a demon at being cheerful.
It's a vulnerable film. These are ordinary people trying to have an ordinary good time. There was a Japanese genre of the 1930s and 1940s called shomin-geki, the drama of the common man, and this is Kurosawa's only example of it. Numasaki and Nakakita are barely scraping by and it's not unlikely that they'll end up having a miserable Sunday. Thus you'll savour their little triumphs. Numasaki plays baseball with some kids, for instance, and smiles! Their darkest hour will be as black as pitch, but fortunately they'll have good times too. I'd have loved to see James Stewart in this film and that's an easy thing to imagine, since Numasaki is giving a Stewart-like everyman performance, albeit obviously not as good as him.
One other noteworthy thing is that these aren't beautiful teenagers. They look ordinary as well as acting it. Nakakita isn't some size zero supermodel, while Numasaki (like everyone else) fought in the war. This isn't just important for the theme, but also necessary for the story because any teenager this obsessed with their financial state and the future would have looked like some kind of freak.
There's also a weird bit near the end. Our heroes go to an empty concert hall and start pretending they can hear the music, which I'd expect to be played today as a comment on their mental fragility. Here it's not. It's about our heroes finding little good things in life, even if they have to make them up from nothing. Furthermore Kurosawa breaks the fourth wall by having Nakakita ask the audience to clap, like Peter Pan asking if we believe in fairies. The man was deranged if he expected any such reaction from a Japanese audience, but apparently at that point in France they'd raise the roof.
This isn't a gut-punch movie and it's doesn't appear to be regarded as one of Kurosawa's masterpieces. His first "major work", or so they say, is apparently Drunken Angel (1948). Instead this is a gentle, honest piece that finds both great darkness and optimism in two ordinary people. It's working hard at its simplicity. There are well-known tunes on the soundtrack, including nursery rhymes. Personally I found it really interesting.