The series follows the adventures of Ben Tennyson,bearer of the Omnitrix,a watch-like device that allows Ben to change into aliens. Storylines alternate between that of Eleven Year Old Ben (a year after the original series) and 16-17 Year Old Ben (a few months after Ben 10: Ultimate Alien). After Gwen leaves for college and Kevin leaves to be close to her,16-year old Ben gets a new partner,a rookie by-the-book alien Plumber named Rook. On a mission to explore a secret underground alien city named Undertown,Ben explores the quirkier side of things in the alien underground and discovers that enemies from his past are looking for a rematch. All the while, Ben becomes targeted by a mysterious hunter,known as Khyber.