In many ways, "Haru No Yume" is like several other very famous films...in particular the French movie "Boudu Saved From Drowning" (later remade as "Down and Out in Beverly Hills") and "My Man Godfrey". See the pictures and you'll see what I mean. All the films are about a stranger entering a rich family's home and you begin to see that the household, though wealthy, is incredibly screwed up and in need of change. Both Bodu and Godfrey provide this change...and in the case of this Japanese film, the man staying with them is far more passive....but the same sorts of family dysfunction abounds.
When the story begins, the old man who sells sweet potatoes is told to go to a rich family's home, as some of the servants want his product. However, soon after arriving, the old man collapses and a doctor is called...a doctor who recommends they keep the man there for a few days until he's recovered. In addition to the screwy rich family, the family and friends of the old men are a mess as well...and want to turn this tragedy to their benefit.
While the story isn't as obviously funny or goofy as the other two movies I mentioned, the Japanese family is a mess and have plenty of crazy soap opera-like stuff going on...though NOT because of the newcomer, as unlike Boudu and Godfrey, this old man is prone almost the entire story and his interactions with the family are much more limited. Instead, goofy family stuff just happens to coincide with his illness. All in all, enjoyable but nothing unique story-wise.
By the way, you might not know it today, but in the 1950s and early 60s, there was a push towards communism and socialism in Japan...hence all the red flags as the workers went out on strike.