In the film, Mulch Diggums is ordered by Commander Root to rescue Holly Short by breaking into Fowl Manor. Once there, however, he is distracted by a wall safe which turns out to contain the most valuable fairy treasure of all: the Aculos. While Holly and Artemis learn of their alliance, Diggums uses this distraction to steal the Aculos, and Commander Root then realizes Mulch's theft. Artemis later enlists Diggums' help when he needs to conduct a theft of his own.
Mulch is the character who narrates the movie, having decided to break the oath of secrecy by telling MI6 what happened.
Diggums decided to turn to a life of crime at an early age. He is a wisecracker always ready with a joke and loves nothing more than a chance to put one over on the Lower Elements Police. His rivalry with Commander Root goes back decades.