Immy has always loved wild swimming; one day, she hopes to become a marine biologist. Tomorrow is the first step towards that goal - completing her entry level diving certificate. But her plans for a good night's sleep are ruined by a strange and vivid dream of a distressed whale in the river. At school she tries to shake it off, but discovers that her nightmare has leaked into reality. Immy and her trusty friend Cosmo must head for the Thames on a mission to save the trapped river whale. Can Immy use her skills to release it from the rubbish-filled nets it's caught in and guide it home?