It's Valentine's day and Caillou is invited to a Valentine's party at his best friend Leo's house. When he gets to the party, he finds a mysterious Valentine card addressed to him! A little investigation back home leads him to his secret admirer: a little sister with a big heart. It is a rainy day and Caillou is disappointed because he will not be able to go play at the park with Grandma. Caillou's mood changes when he discovers an old photo of a younger Grandpa and Grandma dancing along with the first-prize ribbon they won at a dancing contest. With Grandma as his partner, Caillou follows her lead and learns to dance in the living room. When Dad's baseball team, the Tigers, is playing a game at the park, Caillou, Rosie and Mom, go to cheer him on. Although he's too little to go up and bat, Caillou becomes part of the team as the water boy and does a great job. Caillou has to learn that the guest comes first. When Leo comes over for a sleep-over, Caillou sees that Leo misses home a bit. Caillou decides to lend Leo his favorite dinosaur pajamas. The two friends find a perfect solution to make them both happy and have the best sleep-over ever! When Mom and Rosie go to the store, Caillou stays at home with his Daddy, and has a lot of fun imitating him and helping him with the chores.