Doctor Who and Sherlock star Mark Gatiss has secretly joined the cast for The Fantastic Four: First Steps, the 37th film in the Marvel Cinematic Universe.

The sketch (uploaded to YouTube by user Blogster Who) aired April 22. The parody sees Peter Capaldi’s Doctor crossing paths with Benedict Cumberbatch’s Sherlock Holmes.

Sherlock co-creator Mark Gatiss names his favorite episode of the BBC show ahead of its 15th anniversary, and explains why he's not quite celebrating.