Complete story — includes the ending
After a battle against the robots from Dimension 12, Ben, Gwen, and Kevin return to Gwen's house only to encounter Sunny, Gwen and Ben's frivolous and obnoxious cousin whom she used to play with as a child. Gwen's mother Natalie says that Sunny is staying with them for the summer and orders Gwen to supervise her. The team confronts the Dimension 12 robots, and Sunny suddenly joins the battle using mana, revealing that she is an Anodite. At Gwen's house, they are suddenly visited by Antonio, a huge alien and Sunny's boyfriend. Natalie forbids Sunny from seeing Antonio, but she and Antonio escape, pursued by Gwen. Ben tells Gwen that the robots are back, and they arrive at a particle accelerator where the robots have built a bomb. They defeat the robots alongside Antonio and Sunny, but Sunny refuses to follow Gwen's orders to return home, and sheds her human skin, declaring that nobody make her do anything. They battle, and Ben defeats Antonio, while Gwen summons Grandma Verdona to take Sunny and Antonio back home.
Sources and content license
Source and revision history · Adapted from Wikipedia contributors, List of Ben 10: Ultimate Alien episodes. HTML formatting and reference markers removed. · Content license