Caillou's Mommy has just gotten the mail and is disappointed that there are lots of bills. Caillou decides that he wants to give her a letter tomorrow, so that she won't be so sad about the mail. Then his daddy comes in. Caillou tells him about the letter and they both finish it.
When Caillou goes to get an envelope, Gilbert steps on the letter, and Caillou isn't happy. Then his mommy comes in and ask him what he's doing. He says "nothing," then he and his daddy share a laugh.
That night they get the letter ready and put the stamp on it. They have to get extra early the next morning to beat the mailman. They wake up early, give the mailman the letter, and head back in. The mail comes and Caillou's Mommy opens his letter and is very happy and says she will keep it forever. She is impressed that Gilbert signed it, referring to the paw print he left when stepped on it.