Sam has lived seventeen years among the merpeople with his wife Ea. Relations between the two peoples are strained. Sam, now the Minister of the Sea People, doubts that things will ever get peaceful.

One night, the Turtle disappears - the slow freight train between Copenhagen Central Station and the Pearl of the Sea.

Like other teens, Sam and Ea's son, Robin, dreams of getting a bigger tail. After a fight with some classmates, he swims away. Here he notices that there’s something wrong with the rails on the railway and the seabed. It’s a matter of life or death. Everything now depends on Robin.