Isidore Smythe is in danger. He has received threating letters and he does not see a way out. John Angus offers Smythe his help and Smythe is escorted back to his flat. Angus is determined to find out who has sent the letters and he heads out to the former criminal Flambeau and the private detective Father Brown. Before he leaves Smythe’s apartment, he instructs four people to keep an eye on him. Yet when he comes back Smythe is missing and there's a bloodstain on the floor. Is Smythe dead or has he been kidnapped? Who is responsible? Will Angus, Flambeau and Brown manage to solve the case? Find out in "The Invisible Man, a Father Brown Mystery".