Timmy, bored at the beach, sets off with Cosmo and Wanda to explore the deep. Everywhere they go, Cosmo seems to be hated by every single fish. Soon, they end up in the lost city of Atlantis where Cosmo is taken captive when it is revealed that he had sunk the city 9 times. Timmy appeals to Atlantis' leader, King Greg – if he can show good cause why Atlanteans should be happy beneath the sea, Cosmo will be set free. Timmy manages to free Cosmo at the end.