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Sebastian Stan is diving down the rabbit hole on Once Upon a Time and TVGuide.com has the first look at his turn as Alice and Wonderland's Mad Hatter. Once Upon a Time Scoop: Sebastian Stan's Mad role revealed As TVGuide.com first reported, the Captain America and Gossip Girl alum will guest-star on the ABC fairy tale drama. He'll appear in the Wonderland-themed episode "Hat Trick," airing March 25, in which we'll learn how the Mad Hatter became mad. Check out the Mad Hatter's transition into insanity, and see if you can find the major Wonderland Easter egg: Read More > Other Links From TVGuide.com Sebastian StanGossip GirlOnce Upon a TimeCaptain America: The First Avenger