Christmas Eve, 1938, South London. The widowed Mrs Vizzard makes ends meet by renting out rooms at 10 Dulcimer Street: the Jossers occupy the ground floor, the Boons the first, and Connie Coke the top. Mrs Vizzard advertises for a new tenant for the basement.
Doris Josser is chatted up by Percy Boon, but fairground cashier Myrna Watson believes that she and Percy are still an item, something she makes clear when she summons him to see her boss Jimmy. He introduces Percy to Mr Rufus, who has a stolen car he wants "doing up".
The Jossers invite Fred's highly politicised brother Henry to Christmas lunch, which is also gatecrashed by Connie. A dapper Percy comes round to give Doris a present: the mascot from the stolen car. Mrs Josser tries to get rid of him by saying his mother must be lonely, so Percy asks her downstairs as well. Mr Josser reflects with relief that Christmas comes but once a year.
The faintly sinister Henry Squales moves into the basement. Later, Mrs Vizzard goes to the London Psychical Society, where there is excitement (and cynicism) about a new medium, from whom mysterious voices seem to emanate. When the lights go on at the end, she is startled to see that it's her new lodger.
Percy asks Doris out to the Moonraker club. During an interval between dances, a man advises her to leave. A few minutes later, he blows a whistle, triggering a police raid. Percy successfully escapes, but Doris ends up with what she now knows is undercover policeman, Bill Tubbs, who says he'll get her off any charges.
Percy is furious when he's paid a fifth of his fee. Following the car's owner to the cinema, he steals it from the car park, damaging the passenger door. He parks it outside the lock-up and is waylaid by Myrna, who persuades him to take her out for a drive. Percy panics when he sees a policeman, and drives too fast, causing the door to spring open, and Myrna to fall to her death.
The next day, Percy overhears gossip about Myrna's death, but is relieved to hear the suspect is supposedly middle-aged. He relaxes - but is severely rattled when he passes Bill Tubbs, not least because he's taking flowers to Doris. When alone with her, Bill spots the car mascot. Two detectives interrogate Percy, and get a rambling, contradictory alibi.
Mr Chigwell from the Psychical Society tells Mrs Vizzard that Squales is a notorious fraud. Furious, Mrs Vizzard attempts to evict him. Going to pack his things, he discovers evidence that make him realise that Percy's connection with the Myrna Watson case. Feigning illness, he unnerves Mrs Vizzard by ventriloquising a voice claiming that her house bears the mark of Cain.
The police arrest Percy the next morning. Mrs Vizzard is horrified at the damage to her reputation - but is now convinced that Squales is a genuine psychic, and lets him stay. Bill interrogates Squales, who reluctantly agrees to testify against Percy. Doris is furious with Bill for arresting Percy and refuses to speak to him. In his cell, Percy has nightmares about Doris, Myrna, the fairground and his impending fate.
The police subpoena Squales to appear at the Old Bailey. He implores Mrs Vizzard not to go near the trial - but she duly turns up in the public gallery. Percy is found guilty of murder, but the jury asks for mercy - and there's an appeal. Squales' evidence makes Mrs Vizzard realise that he's cheated her again, and she throws him out for good.
Henry goes to see the Jossers, convinced that Percy's case is a symbol of class vengeance. He calls for a public petition and a campaign headquarters is set up in the Jossers' flat. They quickly recruit the services of a religious group, the Junior Guild of Job.
After a fortnight, Henry sets out to Parliament carrying a scroll of signatures and a band of loyal followers. Their flaming torch is quickly extinguished by rain, and various other disasters befall them en route. Crossing Westminster Bridge, they see that Percy has already been reprieved.
It's 31 August 1939. Squales predicts war in six months.
