Dingle the Fool lives in the family home with his two sisters, their husbands, and their infant children. Their mother left the house to all three in equal shares. One sister, Joanna, wants to sell the old place and buy a modern home. The other sister, Dierdre, wants to remain in the house, especially for Dingle's sake.

Dingle plays in the mulberry patch and doesn't seem to understand much. He has a dirty old tennis ball that he believes is full of happiness. One day Dierdre gives in and agrees to sell. When she tells Dingle, he cuts the tennis ball in half, intending to give part of it to his sister.

However, when he sees that the tennis ball is empty (no happiness!), he cries and goes out to climb his favorite tree. Later that night, the house burns down. But Dingle is found safely sleeping in his tree. Joanna and Dierdre face the prospect of a lovely new house, but Dingle has to go to an institution.