After losing his teaching job, John / Johnny, Matthew Rhys encounters his doppelgänger, a failed businessman named Johnny Spence, in a hotel. The encounter sets off a chain of bewildering choices as Spence urges him to drink, and the next morning he wakes up dizzy, without his clothes or wallet, and with Spence gone. George, Pip Torrens, the chauffeur to the Spence family, collects him and struggles to make sense of the tale. He is taken to the family’s grand home, where he is quickly drawn into a web of family and business affairs and must navigate the lives of Spence’s wife, mother, sister, young daughter, and two mistresses. Among them is Nina, Sheridan Smith, one of the women in Spence’s life, while a second mistress, Bela, remains outside the official cast. The family glass business is on the brink of collapse, and Standing improvises by pretending to have signed a contract with a major customer to buy time.
Standing discovers that Paul, Andrew Scott, Spence’s younger brother, is capable but lacks confidence, and that Blanche, Jodhi May, Spence’s sister, harbors a deep resentment toward Johnny, blaming him for a friend’s suicide. Standing also learns that the only money available to save the business is a trust fund belonging to Spence’s neglected wife, Frances, Eileen Atkins. The fund, however, will pay out only if Frances has a male heir or if she predeceases her husband. Spence, curious about what is happening in his absence, returns to retrieve his gun and discovers Standing sleeping with Frances, a revelation he soon uses to his advantage.
While Standing is away at a shooting party lunch with the family, Spence engineers a grim plan: he manipulates Frances into taking an overdose of morphine so he can claim the trust fund and rescue the glass business. She willingly yields, sacrificing herself for the sake of the family. Their daughter, Piglet, sees them together and confronts Standing, still mistaking him for her father. Standing hurries back to the house to find Frances barely alive, and is revived with the help of Charlotte, Phoebe Nicholls, the carer for Spence’s mother.
The confrontation at the glass foundry follows. Spence offers Standing a share of the money, but Standing refuses, and Spence forces him at gunpoint to swap clothing, intending to murder him and dispose of his body in the foundry. A brutal struggle ends with Spence dead, and Standing returns to the house planning to vanish. Charlotte, however, has seen through him and tries to deter his escape. In the final scene, Standing is shown with the Spence family, including a pregnant Frances, as they watch the Coronation on a newly acquired television set, a quiet counterpoint to the violent upheaval that preceded it.
