Ex-barrister William Fitch stands trial for writing a series of fraudulent letters, requesting money from a number of charitable bodies. Claude Babbington, an incompetent, upper-class lawyer, acts as prosecutor, but his bungling and Fitch's brazen guile result in the case being thrown out of court.
Over a drink in a nearby bar, Fitch and Babbington agree to a partnership, but their plans are interrupted by Grimshaw, a convicted forger who swears a fatal revenge on all those responsible for his prison term. He informs his two 'learned friends' that the judge who sentenced him was first on his list of six names, and that Fitch, his ineffective defense lawyer, is pencilled in as his final victim.
Perturbed, Fitch and Babbington attend the dead judge's inquest but, after intervening in the wrong hearing, their attempts to persuade the court that Grimshaw was the killer are met with fury and the couple are ejected from the proceedings.
Grimshaw reappears and warns them not to contact the police otherwise Fitch's retribution will be brought forward. Terrified, they privately agree to track the murderer down and resolve to act on his cryptic clue that 'Safety' Wilson, a small-time crook who turned King's evidence at his trial, will be the next to die. They track Wilson down to an insalubrious bar in Stepney, but he laughs at the threat and lures Fitch into a game of crooked poker. However, the ex-barrister expertly cheats and arouses Wilson's fury.
In the barroom, Babbington becomes embroiled in a fight and during the melee Grimshaw is able to murder Wilson. Outside, he gives Fitch another clue, this time indicating that his next victim is to be his treacherous ex-girlfriend, now starring in a production of Aladdin in Lancashire. Fitch and Babbington race to the theatre where she performs, but despite becoming involved in the pantomime and warning her of the peril, are unable to prevent her murder.
Again, the killer confronts the couple and implies that the next name on his list is Doctor Scudamore, the psychiatrist who refused to testify that he was mentally unbalanced. The intended victim now runs Maybury Court, a hospital for the insane, but Grimshaw has supplied one of the patients with a saw with which to create a 'tiger trap' - a hole in the floor camouflaged with a rug. Fitch and Babbington arrive just as - inevitably - the lunatics are taking over the asylum. Conned by an inmate, they feign madness, only to find the doctor unwilling to believe they are sane. Before they are able to warn him of the threat he falls through the trap to his death.
Thoroughly dejected, Fitch and Babbington arrive at the latter's home to find Grimshaw has been helping himself to his alcohol and cigarettes. In return, he has left them a note, and Fitch deduces that he plans to blow up the House of Lords in revenge for their refusal to accept his appeal. The two race to Palace of Westminster and, disguised as Beefeaters, search for explosives. They discover boxes of dynamite in the vaults, but become locked in the chambers, only able to escape to the top of St. Stephen's Tower. Here they confront Grimshaw for the final time, with the madman informing him that when Big Ben strikes noon, a device will be triggered leading to a literally explosive act of vengeance which will kill the peers below.
Fitch and Babbington evade him and, outside the tower, scale the clock-face and prevent the minute hand reaching the fateful twelve. Following a precarious tussle with Grimshaw, the police arrive and arrest the murderer, but Fitch and Babbington are left dangling from the tower, reflecting that their only problem remains extricating themselves from their current predicament.
