The Visitor is the thirty ninth episode of Ben 10 and the third season finale.
Two snowmobilers accidentally release an alien named Xylene from a strange container. Xylene locates the Omnitrix and attacks Ben, under the impression that he had stolen it. However, when Grandpa Max comes to defend, the two recognize each other as old friends. Xylene and Max reminisce later that night. Ben as Wildvine spies on them, catching Xylene asking Max to leave Earth and travel through the universe with her. Distraught, Ben runs back to the campsite, where he's unable to ward off an attack by one of Vilgax's remaining drones which has upgraded itself using parts from the Rust Bucket and has to rely on Max and Xylene for help. There, Xylene reveals that she was the commander of the ship bringing the Omnitrix to Earth when Vilgax attacked it, and that she had intended to send it to Max; Ben received the Omnitrix by accident. Ben runs off in shock and Gwen follows. The drone attacks again and captures Max.
Ben and Xylene reluctantly team up to save him. They track the drone to the docks where it has assembled technology from Max's Plumber suit, and a large quota of military grade tech. In the process of defeating the mecha, Xylene uses her knowledge of the Omnitrix to unlock a new form, Perk Upchuck, who destroys the robot by literally chewing it up and spitting it back out. At that, Xylene prepares to leave Earth and repeats her offer to Max. Max declines, claiming that Ben and Gwen are his whole world right now.
The gang learned that the Omnitrix was meant to be sent to Grandpa Max, not Ben.