Ben Tennyson's cruise vacation takes a turn for the weird when he discovers that his mysterious new friend Lorelai is actually a mermaid, the princess and head protector of an alien-hybrid undersea kingdom called TerraPisciss. The mer-people are heroes of the sea and have never needed help before-- so why are they suddenly bringing Ben 10 and the Plumbers into their trust? Action, aliens and romance on the high seas!