Ben Tennyson seeks to prove himself as a member of the Justice League to Superman, Batman, Wonder Woman and Aquaman as they seek to face a threat that is familiar to both of them.
- Ben Tennyson (Yuri Lowenthal) - gets put on the Junior Justice League until he's ready, and makes fast friends with Supergirl and Static, but has a tense rivalry with Stargirl and Guy Gardner
- Justice League:
- Clark Kent/Superman (George Newbern)
- Bruce Wayne/Batman (Kevin Conroy)
- Diana Prince/Wonder Woman (Susan Eisenberg)
- Arthur Curry/Aquaman (Cary Elwes)
- Jefferson Pierce/Black Lightning (Arif Kinchin)
- Carter Hall/Hawkman (James Remar)
- Kimiyo Hoshi/Dr. Light (Lauren Tom)
- Daniel Cassidy/Blue Devil (David Sobolov)
- Guy Gardner/Green Lantern (David Oyelewo) - jealous of Ben because he reminded him of a younger version of Hal Jordan
- Junior Justice League Squad:
- Kara In-Ze/Supergirl (Nicholle Tom) - she and Ben have a friendship bonding over their cousins
- Courtney Whitmore/Stargirl (Giselle Loren) - jealous of Ben's good looks and how popular he is
- Virgil Hawkins/Static (Phil LaMarr) - befriends Ben
- Irwin Scwab/Ambush Bug (Ahmed Bhest)
- Kevin Levin (Greg Cipes)
- Gwenniac (Ashley Johnson/Corey Burton) - the film's main antagonist, a fusion of the Robot and Brainiac, her/his goal is to get revenge on Kevin for making her look like Ben as a drag queen, and to remove herself from Ben's stupidity, her appearance is the robot's appearance but with Brainiac's feature.
- Amanda Waller (C.C.H. Pounder) - the film's secondary antagonist, after Professor Paradox's death, she has trying to gain Ben's Omnitrix so she can use the aliens to recreate another Ultimen.
- King Faraday (Scott Patterson) - the film's tertiary antagonist, chases after Ben after he was framed into sneaking into Cadmus headquarters, only to be arrested for lying about Ben's infiltration.
- The Intergang - the quitarnary/quinary antagonists, hired by Waller to capture Ben.
- Heatblast (Steven Blum) - used to save Guy from Intergang.
- Swampfire (Daniel Riordan) - used to help Blue Devil escape from Cadmus.
- Diamondhead (Jim Ward) - used to destroy the Robot Gweniac for good.
- Terraspin (Bill Fagerbakke) - used to keep Batman from drowning.
- Grey Matter (Richard Steven Horvitz) - used to defeat Waller.
- This crossover is dedicated to the memory of Dwayne McDuffie, who died on 2011.