Isabel and Zoë love playing with Isabel's puppy, Rollo-- especially when they play at Zoë's house because that's where the Baby Stealer (aka Zoë's older brother, Lionel) lives. The girls push Rollo around in a stroller until the Baby Stealer jumps out and scares the girls, then runs off with Rollo. It's a thrilling game that always has a happy ending-- with two giggling girls reunited with their fur baby, safe and sound. But one day, Lionel won't play the game. In fact, lately all he seems to want to do is play video games with his friends or raid the fridge for snacks. When anyone tries to talk to him, he either answers with a grunt or not at all. "What is wrong with him?" Izzy asks. "He's a teenager," Zoë explains. As if that isn't worrying enough, Izzy has been noticing changes in Rollo, too. He's not fitting into Izzy's baby clothes anymore, and he keeps doing things that get him into a very big amount of trouble, like eating everyone's ice cream when they aren't looking. His shenanigans get worse and worse until Isabel finally realizes something-- Rollo isn't a baby anymore-- he's a teenager! Is there a cure for being a teenager? Izzy is going to find out!