Father Brown (Mark Williams) arrives at a police event and runs into Inspector Mallory (Jack Deam) who clashes with his former boss DCI Webb (Colin Mace). Mrs McCarthy (Sorcha Cusack) is shocked to see dioramas of murder scenes - what sick mind comes up with something like this? Wilber Lesser (David Yelland) says his daughter Agnes (Mariah Gale) does, as a teaching tool in the art of crime detection, they're remarkable. Later Agnes takes Father Brown to her workshop and shows him a diorama depicting the death of her mother, who fell down the stairs. When Agnes displays it for all to see, Wilbur loses his temper. As everyone files away DCI Webb, who was the original investigating officer, spots something in the diorama and takes it. Soon after he's found dead, a carpenter's awl stabbed into his chest. With blood found on his coat, the prime suspect is Inspector Mallory himself, who is arrested for murder...