Sir Giles and Lady Maud Lynchwood are not the most happily married of aristocrats. She wants an heir but he cheats on her with incompetent dominatrix Mrs Forthby. The pair agree to divorce but, to stop Maud hanging onto her ancestral home Handyman Hall Giles approaches the Ministry of Transport, suggesting the house be demolished to make way for a motorway. At a public meeting Lady Maud, supported by Blott, her gardener, whips up local support against the motorway, leading to a riot with the destruction of buildings and her ultimate arrest.

In the wake of the riot naive civil servant Dundridge is sent to Worford by devious ministers as the PM opposes any other route for the motorway than Handyman Hall. Released from prison Lady Maud entertains him behind her husband's back, charming and flattering him and she is much cheered when he agrees that the motorway should take the form of a tunnel going under the grounds and not through them. A dismayed Sir Giles is all too happy for his solicitor to pay a young lady to get Dundridge drunk and compromise him.

Purporting to side with his wife, Sir Giles uses the compromising photos of Dundridge to dissuade him from his tunnel idea and to run the motorway through the alternative route, the new town of Otterton. Thus, at the resumed hearing, Dundridge leans on convenor Lord Leakham -using Lady Maud's threats to sue him for having her falsely imprisoned - to save the hall. He is, reluctantly, put in charge with supervising the construction of the new route. A grateful Lady Maud invites him to dinner and contrives to have him stay the night but, after being pursued by an amorous lady and shot at by Blott, "defending her honour", an irate and nude Dundridge vows to run the motorway through Handyman Hall after all.

Enraged at Lady Maud's treatment of him, Dundridge approves the motorway going through Handyman Hall and the surrounding Clene Gorge. Lady Maud suspects that, as he and not she will receive the compensation money, Sir Giles is in favour of this and finding the compromising photographs of Dundridge confirms her suspicion. She decides to open a safari park with an electric fence to keep people out and also, using Blott's talent for mimicry, persuades Giles' broker to sell all his shares. She then tracks down Mrs Forthby, with whose help she gets Blott to photograph one of Sir Giles' S&M sessions, threatening to publish the evidence unless he signs a divorce settlement agreeing to the motorway being diverted elsewhere. She feels triumphant but, on returning home, hears blasting in the gorge.

Outraged at the demolition of the gorge, Lady Maud goes to see Dundridge, suggesting that they work together to expose Sir Giles as the blackmailer. However, since Dundridge detests her and believes that she was responsible for compromising him, Dundridge refuses, making it his mission to destroy her home. That night Blott gets one of the workmen extremely drunk and encourages him to cause destruction - and a death - in the village to increase public hostility to the roadworks. Sir Giles, meanwhile, who is living with Mrs Forthby, drugs her and returns to the hall, which he tries to burn down, escaping into the grounds after a fight with his wife.

Attempting to flee the grounds Sir Giles is pounced upon and killed by a lion, his offshore investments leaving Lady Maud a very rich widow. Blott, meanwhile, who has squirrelled away a substantial amount of ammunition, barricades himself in the gatehouse to the hall as Dundridge orders the SAS to attack. An emotional television broadcast in which Blott insists that an Englishman's home is his castle helps put paid to all further ideas of the motorway whilst Dundridge is blamed for everything and sent to prison where he enjoys himself reorganizing the library system. Appreciating all that he has done for her, Lady Maud marries Blott, putting him up as a Member of Parliament and they look forward to creating an heir.