On last night's terrific Once Upon a Time episode, viewers met a new nemesis for Rumplestiltskin.

On next Sunday's "The Doctor," meanwhile, we'll spend time with an old favorite and finally learn the answer to one of the show's more pressing questions: Who is Dr. Whale?

Unfortunately, the official ABC teaser for this installment appears to reveal his identity. Consider yourself very much spoiler warned and watch now, as Whale works with Regina on a most unexpected patient...

For more sneak peeks from "The Doctor," browse through our collection of Once Upon a Time photos from the episode.

Matt Richenthal is the Editor in Chief of TV Fanatic. Follow him on Twitter and on Google+.