Alison Jenner, the mistress of a large rich estate, is being blackmailed. An unknown person sends her threatening letters, hinting that she is a murderer. Fearing that they will find out and his wife's good name will suffer, Jeremy Jenner invites a familiar couple, Meredith Mitchell and Superintendent Alan Markby, to visit and asks them to unofficially investigate. Markby is not ready to get involved in this matter - it seems far-fetched to him. But when the body of Jenner's daughter Fiona is found in the lake, he can no longer stay away.