Gwen is abducted through a portal, so Ben changes to XLR8 and follows after her. They find that Gwen's adult self came back in time to bring Ben into the future, saying that, although Ben's adult self has mastered the Omnitrix, he still needs some help that only his younger self can provide. It's not, however, the combined threat of Dr. Animo and Vilgax that she worries about.
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It's great seeing you, too, but the same shirt? Grandpa, it was twenty years out of style twenty years ago.
Old Max Tennyson
Hey, when you find a look that works...