Season 4 Episode 4

Ventriloquist Uncle Mirth invites his brothers George and Mortimer, Father Brown and others to a party.  But, oddly, they find themselves in a derelict building with no apparent exit.  George and Mortimer are killed and it would appear that Uncle Mirth wants revenge on those who had him committed to an insane asylum two years earlier. The survivors escape but Father Brown suspects that one of them may be in league with, or manipulating, Uncle Mirth for their own ends.