In 2023 London, Detective Sergeant Shahara Hasan spots a young Asian, Syed, lurking near a far-right demonstration and holding a gun. She chases him and finds a naked, shot corpse in Longharvest Lane. She eventually traces Syed and tries persuading him to surrender but, distraught and protesting innocence, he kills himself.

In 1941, a mysterious phone call instructs Detective Sergeant Charles Whiteman to retrieve and dispose of an (identical) body from Longharvest Lane. He is followed by Farrell, a suspicious superior but, on the point of finding Whiteman with the body, Farrell is killed in an air-raid. Whiteman is put in charge of investigating the body and Farrell's death, while the caller upbraids him for failure and demands he sabotage the investigation.

In 1890, Detective Inspector Alfred Hillinghead investigates the same body in Longharvest Lane. He obtains photographs of the crime scene from gay newspaper photographer Henry Ashe, who was suspiciously nearby. Seeing a man in one of the photos, the coroner brusquely warns Hillinghead against further investigation.

Hasan's team investigates Elias Mannix, a troubled teenage friend of Syed's who was seen with him before the body was found. Hasan interrogates Mannix's foster parents, who lure police into a trap which kills her colleague. Mannix's foster mother cryptically thanks Hasan for what she will do for Elias, then regrets saying anything and bites her own tongue off.

Whiteman frames a local criminal for the crimes, although his superior DCI Calloway remains suspicious. A child who previously saw Whiteman with the body at Longharvest Lane arrives at his station.

Hillinghead is encouraged by his superior to close the case, but does not burn the photo of the man in the glass reflection. He and Ashe reluctantly agree to further investigate the case unofficially.

Hillinghead and Ashe identify the man in the photo as stockbroker Sir Julian Harker. Harker invites Hillinghead to one of his mother's seances, where Hillinghead is drugged by the Harkers and photographed in a compromising position with Defoe's corpse. Hillinghead warns Ashe that Harker is plotting something and the two sleep together. Later Hillinghead returns to Longharvest Lane and carves his name and the three-pronged symbol in the base of a wall.

The child, Esther, attempts to blackmail Whiteman. The organisation instructs him to kill her. However, Whiteman cannot bring himself to kill Esther, so he fakes her death and hides her in his flat.

Elias Mannix is arrested and tells Hasan that his foster parents have knowledge of future events, including of an explosion they claim he will carry out. Back in Longharvest Lane, Hasan notices Hillinghead's name carved in the wall.

Ashe notices a fingerprint Harker left on Hillinghead's spectacles, meaning they can still link him to the body via other prints Hillinghead previously found on Longharvest Lane.

Whiteman and Esther escape to an air raid shelter during a bombing, where Whiteman is assailed by and kills a fellow policeman who had been following him. The woman who has been calling Whiteman from the organisation is revealed to be Polly, Hillinghead's now elderly daughter, and poisons Esther.

Hasan discovers archive case files from Hillinghead and Whiteman's investigations, as well as links between Mannix's foster parents, the Morleys, and the Harker family. Hasan breaks into Harker's abandoned home, where Andrew Morley speaks cryptically about being part of a large group on an inevitable path, whose roles are to change the world and future. Morley then locks Hasan in an underground room.

Hillinghead brings Harker in for questioning about Defoe's body, but Harker blackmails him with the compromising photographs, telling him to frame Ashe for the murder.

Whiteman secures DCI Calloway's help in finding Polly, discovering she is Managing Director of Harker Bank. Whiteman abducts Polly, threatening her with Russian roulette until she confesses to killing Esther. Calloway arrests both of them for murder, but is then killed by Polly's son Hayden, a senior police officer.

After learning that Elias' fingerprints match Julian Harker's, Hasan tracks down Elias' birthmother, Sarah Mannix. She learns DCI Barber is Harker's great-grandson, and Elias' father, but Barber disappears after taking Elias from HM Prison Feltham. Hasan finds a 1941 recording addressed to her from Harker, confirming that she will help him bomb London.

Hillinghead informs Ashe that he will be convicted for Defoe's death, but Ashe refuses to flee. Hillinghead signs a false confession, tells his family of his sexuality and is arrested. Polly maintains his innocence.

A now elderly Harker makes recordings addressed to others. Whiteman refuses Hayden's offer of joining the conspiracy and kills Hayden, Polly and Julian in revenge for Esther's death. He is arrested, convicted, and sentenced to death by hanging.

Hasan informs SO15 of the bomb. DI Bothroyd accompanies her to Harker Bank where they find a vault containing a nuclear bomb. Hasan meets Barber and Elias, who holds the detonator. Bothroyd kills Barber. Later, Hasan takes Elias to his mother, but she shuns him. Pushed over the edge, Elias changes his mind and calls a number Barber gave him, which triggers the explosion.

In 1889, Mannix adopts the identity of Sir Julian Harker, a veteran who supposedly died in Mandalay. Lady Harker agrees to go along with the deception as Mannix promises fortune, family, and prestige. In 1890, Harker is photographed by Ashe. He purchases the bank and oversees construction of the bomb vault, grows his cult, and romances Polly. Harker has Hillinghead killed, after which he marries Polly and sires a son, Hayden. Polly is brought into the conspiracy.

Maplewood manages to tell Hillinghead about Harker's past and Hillinghead's impending death. Hillinghead tells Harker he knows the latter is Elias Mannix and that he will die alone and unloved. The confrontation changes the timeline, as the shaken Harker becomes cold and distant. He tells an anxious Polly that she is bound by the time loop, and their relationship deteriorates.

In 1941, a regretful Harker makes a secret last recording advising Elias against the plan. After Whiteman kills Polly, Harker convinces him to get the recording to Hasan. Whiteman hides the recording in a framed photograph before being killed by Hayden's officers.

Hasan travels back to 2023 and finds the recording left by Whiteman. She informs her younger self of the second trigger. Older Hasan then plays Harker's new recording for Elias to hear, and he does not detonate the bomb. Sarah embraces Elias, who disappears from existence along with older Hasan as the loop is undone.