Despite Inspector Sullivan that he mind his own business, Father Brown can't help himself as Sid Carter is arrested after seeing a man trying to dispose of a body in a nearby river. Sid was trespassing on private land but the dead man, Archie Waterson, is reported in the press as having drowned. Waterson was working on a restoration project for the somewhat mad Lord Masters but Father Brown knows something is amiss - the real Archie Waterson died as an infant many years before and is buried in churchyard. Sullivan had good reason to tell him to mind his business as Brown finds himself embroiled in the secret world of MI5, something Lady Felicia seems to know about. She also knows the man in charge, Daniel Whittaker, though she thoroughly dislikes the man. Solving the crime will first require the uncovering of a Soviet spy however.
—garykmcd