Ken and Harry are guarding Upper Ridley Hall, near where Harry lives, and where a big Society wedding will be taking place between Sir Alan and Lady Tessa Bolton's daughter, Kate, and Rupert de Borchgrave, a Belgian Count.

Sir Alan asks Ken to keep a discrete watch on one of his houseguests, Johnny Hunter, a suave gentleman who drives an Austin Healey sports car. Ken senses a tension in the house: why is Hunter nosing around the house and taking such an interest in Kate, and why is Lady Tessa so evasive about him. Why is Kate secretly making furtive phone calls and who is the man who she meets late at night?

While patrolling the grounds, Harry spots two men behaving very suspiciously and stalking one of Sir Alan's stags. He reports this to Philip Braithwaite, Sir Alan's Estate Manager, who doesn't seem to take the matter seriously. Harry suspects that Braithwaite himself might be involved in the stag poaching. He follows one of the estate vans to a ruined chapel which he discovers the poachers are using as a storeroom.

Ken sees Lady Tessa wrapping some of the family's paintings in a brown paper and later hears Johnny threatening her and walking away with the same brown paper package. Realising that there is something seriously wrong, he challenges Lady Tessa. At first she denies everything but after Ken tells her that he has seen Hunter selling the paintings at an antique shop she admits that he is blackmailing her: he is Kate's father and is threatening to tell Sir Alan. Ken witnesses Hunter blackmailing Lady Tessa and threatens to hand over his evidence to the police unless Hunter leaves the family alone. But Hunter has one final trick up his sleeve: he tries to blackmail de Borchgrave, threatening to tell Sir Alan who will cancel the wedding and stop de Borchgrave getting his hands on Kate's money. However de Borchgrave, a homosexual who was only marrying Kate for her money, cancels the wedding when he learns that Kate will no longer inherit Sir Alan's money.

Laura and Rocky take Kate to have her wedding dress fitted but on the way back she tricks Rocky into coming with her to the ruined chapel. Here, on the night before the big day, she gets him to "officiate" at a mock wedding between her and Philip Braithwaite with whom she has been having an affair. But Ken and Harry are also watching the chapel, waiting for the stag poachers to return.

Kate and Philip elope in her Morris Traveller. When Hunter turns up to make his big revelation, Sir Alan admits that he has always known that he was not Kate's father but he still intends to leave his house and money to her.