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-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 Under town, 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.