en was all set to be a solo hero with his new Omnitrix after Gwen and Kevin left for college, but Grandpa Max teamed him up with a rookie, by-the-book partner. Together they explore a secret alien city, known as Undertown beneath the city of Bellwood. Meanwhile, a mysterious hunter, known as Khyber, sets his sights on Ben! With all new alien heroes to choose from, the fun is just getting started. It's a whole new Omniverse!

Overview from themoviedb.org