The Doctor had better watch out. Emmy and Tony Award-winning actor Neil Patrick Harris is joining Doctor Who, and not as one of the good guys.

Once and future Doctor Who showrunner Russell T. Davies posted a photo of Harris in costume to his Instagram on Monday. Harris sports a severe gray haircut, pince-nez glasses, and what looks like a shop apron over a button-up shirt and floppy tie.

"Neil Patrick Harris, welcome to Cardiff!" Davies wrote in the Instagram post's caption. "Playing the greatest enemy the Doctor has ever faced. Such a great actor, such a great man, it's an honour and a hoot. Have fun!"

Harris also posted the photo to Instagram, writing, "My current gig. Never looked more dashing. Thank you for inviting me into your Whoniverse, Russell T. Davies. I'll try my hardest to do my worst. This Doctor has no idea what's in store. And even if he does... Who cares? Ha ha ha HA ha-ha-ha!"

Of course, Doctor Who fans want to know just what that "greatest enemy" teaser means, and who exactly Harris will be portraying. But that info is all still confidential.

"It's my huge honour to open our studio doors for the mighty Neil Patrick Harris... but who, why, what is he playing?" Davies wrote in a statement. "You'll just have to wait. But I promise you, the stuff we're shooting now is off the scale. Doctor beware!"

Davies served as Doctor Who showrunner from 2005 to 2010, and will return to the show in that role in 2023. 

The role of the Doctor is getting a new acto,r too. Ncuti Gatwa will play the role of the 14th Doctor, the BBC announced in May. He'll be the first Black actor to play the role, replacing Jodie Whittaker, the first woman to play the role. No specific dates have been announced for either Gatwa's or Harris' first appearance.