Don't watch Ikiru if you're already down on life. While it is not a bleak film, it sure starts put that way. This is more for someone in a pensive mood who is curious how a person (in the 1950's no less) would respond to the news that they have six months to live. It's hard not to think how you would answer those same questions.
The main character, Watanabe, isn't even given the prognosis directly. A stranger recognizes his symptoms and anticipates the doctor's vague prescriptions. The doctor will say "ulcer" when he means "stomach cancer." This is a scary thought all on its own, but some societies cannot tolerate the disruption that a dying person could cause. So, yeah, it's not a stretch that this could happen. Only thing worse than having six months to live is that no one bothered to tell you.
As the nameless narrator points out, Watanabe isn't doing much living at the moment. He is a widower who appears to be in his early 50's (the actor was 47) with an adult son and an irritating daughter-in-law. He has been working at the same municipal job for 30 years, probably with a few changes in title but no significant responsibilities. He eats nothing but plain noodle soup for lunch. When a co-worker shares a comic about people never missing work so no one knows how unnecessary they are, Watanabe is hit hard. It wasn't directed at him but it might as well have been.
He feels the need to share this bad news with someone. He tries to get drunk and meets up with a jaded author (Yunosuke Ito), practically begging the guy to help him blow a large amount of money in one night. The author will not take advantage, but still takes him out to the clubs promising "tonight is on me." They go to a variety of places although it doesn't seem to do either of them much good. The author spends much of the night in a drunken ramble and Watanabe brings down one party with his rendition of "Life is Brief." He doesn't get that mortality is one issue these revelers specifically want to avoid. He doesn't cross paths with the author again, but he does get a new hat out of the experience.
Soon after his failed attempt at debauchery, he hears from a bubbly co-worker Toyo (Miki Odagiri) who has no intention of repeating his life. She tracks him down outside of work for a signature and unwittingly cheers him up with her presence. His family is now starting to get suspicious, with the new young friend and the new hat (such nerve!), so Watanabe still doesn't feel comfortable sharing his diagnosis. Toyo doesn't want anything from him, even defending his son when Watanabe complains about him. She grows weary with him and eventually drops out of the picture as well, although in their last scene together she offers the best advice she can at her stage in life. Find something that you love and do it. Simple, but what he needed to hear.
He returns to work and sets his mind on one project - building a new park at a mosquito-infested area making kids sick. The idea's been floating around all movie, but no one wants to take any initiative. He has no reservations any longer. Then there is a five month fast-forward and our hero is gone. Several of Watanabe's co-workers, including subordinates, equals, and superiors come by to pay their respects. The new park is a focal point of the conversation and there are many opinions of how vital he was in the effort. When they can't settle that point, they start discussing whether he knew he would die (he had never even told his son) and eventually surmise that he did from pieces of conversation. After the brass leaves, the younger workers, now quite drunk, vow to continue Watanabe's work and not get bogged down with bureaucracy any longer.
You can guess how that worked out.
Takashi Shimura lived for another 30 years after this film, becoming one of Japan's most recognizable faces. Just two years later he would appear in two iconic films, The Seven Samurai and Godzilla. This film is often considered his best as he is in nearly every scene and in a few doesn't even speak. The supporting actors, the author and Toyo, do good work in hearing his anguish. If you could distill the film into just three words, they would be "Make a difference." Life really is brief.