Sebastian Stan and Jennifer Morrison

Gossip Girl's Sebastian Stan will finally make his long-awaited debut as Alice in Wonderland's Mad Hatter on this Sunday's episode of Once Upon a Time. And we've got an exclusive sneak peek!

First Look: Sebastian Stan goes Mad on Once Upon a Time

While we'll learn how the Mad Hatter became, well, mad in Wonderland, his real-world counterpart, Jefferson, is just as eccentric. Surprisingly, his kidnapping and drugging of Emma (Jennifer Morrison) is the least of his offenses.

Once Upon a Time airs Sundays at 8/7c on ABC. Get your first look at Stan's debut below: