Lights, camera, action!
There are two key plotlines in You Can't Take It With You. They're each important and interesting in their own right, but they're also so cleverly tied together that at the end you get a big happy bow. Frank Capra loves big happy bows. He wore them everywhere…. or he should have.
The first key plot involves the big bad munitions manufacturer, Mr. Kirby. Mr. Kirby wants to get a munitions monopoly, because alliteration, and also money. To do that he needs to buy up all the land around the factory of his chief competitor, Mr. Ramsey.
He does that thing, but there's one holdout: Grandpa Martin Vanderhof, an eccentric old coot. Vanderhof owns his house and loves his house (his wife died there) and he doesn't care about money, so he's not going to sell, no matter how many times Kirby's agent asks him to.
The second key plot involves Kirby's son, Tony, a vice-president at the bank, and Alice Sycamore, Tony's secretary. Alice isn't just Tony's secretary, though; she's also Grandpa Vanderhof's granddaughter. (The plot thickens.)
Tony is in love with Alice; Alice is in love with Tony. One thing leads to another, and then they're engaged. Unfortunately, the snobby Kirbys don't much like the idea of Tony marrying a secretary. How would they feel if they knew that secretary's relations were thwarting their business deals? Well, that's what you're here to find out.
To try to get Tony's parents to approve of a marriage, Alice invites them over for dinner. Tony gets cute though and brings his parents on the wrong night, so that they'll see Alice's family as they really are (note to prospective suitors of all genders: don't get cute.)
The dinner is a disaster.
For one thing, the Sycamore family is extremely eccentric and goofy and generally not fit for polite company. Alice's sister dances all over the place, her mother is always writing on her typewriter, and her dad is making fireworks in the basement. Mr. Kirby is wrestled to the ground by the Russian dance teacher Kolenkhov, who always hangs out there because that's the sort of person who hangs out in the Sycamore place.
Then—because it just needed to get worse—the house is raided by police who have misinterpreted some party invites as seditious literature.
Then all the fireworks go off.
Then everybody goes to jail.
Mrs. Kirby is understandably upset by this turn of events. But she's also kind of a jerk. The upshot is, while waiting in prison, she sneers at Alice for being a lowborn secretary and trying to steal her Tony for his money. Alice is peeved and uses her kung fu abilities and there is a massive fight scene with ninjas.
Okay, actually, no ninjas. But Alice is angry. When they all go before the judge, Mr. Kirby lies about why they were all at the house because he's embarrassed to have Tony linked to the Sycamores. Alice shouts out the truth, though, and there's a big hullabaloo (though still no ninjas). Tony tries to tell his parents they're being jerks, but Alice feels he should have done that a lot earlier. She breaks off the engagement and runs out of the courtroom.
The news is in all the papers (there's a coming war in Europe and a Depression on, but apparently people in Capra's world are really interested in the domestic affairs of munitions manufacturers). Because of the scandal, Alice leaves town, and Tony doesn't know where she is. Grandpa decides to sell the house and move to where Alice is so they can all be together.
Alas, true love is over, and the people on the street will have their homes replaced by munitions factories.
But then a miracle occurs (as they tend to in Capra films).
Tony tells Mr. Kirby that he's going to quit the bank and go study grass (no, really). Mr. Kirby realizes his whole life is hollow and pointless, and he quits. He goes to the Sycamore house and plays the harmonica. Tony is there trying to find Alice, and Alice returns suddenly to tell her dad not to sell the house. The lovers reunite and reconcile; Mr. Kirby learns the meaning of happiness. Even Mrs. Kirby seems like she's going to come around.
And now that Capra's gotten everyone to the point where they're happy, he's ready to say "The End."