Ned Hannigan, a notorious child killer, is found dead in the woods after serving 14 years for confessing to the murder of his childhood girlfriend 14 year old Maggie Banks who was besotted with him. Inspector Mallory arrests Maggie's mother after finding fingerprint evidence. Maggie's father, John, had already attacked Hannigan and her brother, Daniel, had visited him, both now adults, just before his release from prison. Hannigan had never revealed where the body was. Father Brown wonders why Hannigan returned to Kembleford and who he was meeting and his only clue in his prison bag was a wooden symbol of the star system Gemini..