It’s Valentine’s day and Caillou is invited to a Valentine’s party at his best friend Leo’s house. Caillou is excited and with Mom’s help, he prepares special Valentine cards for his friends. When he gets to the party and hands out his Valentine’s cards, he finds a mysterious Valentine card addressed to him! It’s not from anyone at the party. A little investigation back home leads him to his secret admirer: a little sister with a big heart.