When Phineas and Ferb's father shows them how to use a new snow cone machine, the boys are inspired with their next big idea: they'll make winter in the middle of summer! Soon, their backyard is a snowy "swinter" wonderland with all their friends skiing, snowboarding, and sledding. Then, later in the summer, Isabella gets her tonsils out and can only eat ice cream. The boys cheer her up by building a huge ice cream maker to make her the biggest sundae ever!