Sebastian Stan

Sebastian Stan is going mad on Once Upon a Time.

As TVGuide.com first reported, the Captain America and Gossip Girl alum will guest-star in an upcoming episode of the ABC fairy tale drama. Details have now surfaced on his top secret role: He'll be playing Alice in Wonderland's Mad Hatter. "In that episode, we find out how the Mad Hatter became mad," executive producer Edward Kitsis says, noting that Stan has been fantastic in the role.

Exclusive: Captain America's Sebastian Stan heads to Once Upon a Time

Like the other tales the series has adapted, there will be a Once spin on Wonderland. "The first difference you'll notice is who the Mad Hatter was and his backstory. Then what Wonderland is and the changes of it is something we hope to explore also. We're hoping this tells you the story of the Mad Hatter, but also will leave you with questions about Wonderland."

In the Wonderland-themed episode titled "Hat Trick," which airs March 25, Roger Daltrey of The Who is set to voice the hookah-smoking Caterpillar.

Once Upon a Time airs Sundays at 8/7c on ABC.