In 1941, Detective Sergeant Charles Whiteman faces antisemitism from his colleagues during World War II. He also works for a mysterious organisation that sends him orders by telephone, ending each call with “know you are loved”. They instruct him to retrieve an identical body from Longharvest Lane. Transporting the body, he is stopped by DI Farrell, who suspects him of corruption. However, Farrell is killed during a German bombing raid before he can search the car. Whiteman escapes, leaving the body, and is later put in charge of investigating it and Farrell’s death. The organisation, who wanted the body, demand he sabotage the investigation and destroy all records.

In 1890, Detective Inspector Alfred Hillinghead attends Longharvest Lane when the same body is discovered. He obtains photographs taken of the body from local journalist Henry Ashe, who he also arrests for gross indecency upon discovering his homosexuality. In one of the photos is a window reflection of a man hiding near the body. The coroner, Ladbroke, becomes scared on recognising the man. He warns Hillinghead to burn the investigation.

Whiteman frames a local rapist and black market criminal for the body and death of DI Farrell, 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 cannot bring himself to burn the photo of the man in the glass reflection. He is visited by Ashe, who wants the photo back. The two reluctantly agree to further investigate the case unofficially.

The child, Esther, blackmails Whiteman in Yiddish to be paid off otherwise she will tell his superiors of his involvement with the body. The organisation instructs him to kill her, and have them followed. Whiteman cannot bring himself to kill Esther, so fakes her death and hides her in his flat.

Elias Mannix is arrested and tells Hasan his foster parents "know things" about future events. They have told him similar bodies have appeared before, and that he will be responsible for a devastating explosion which Hasan will make him carry out. Her superiors ignore this, and remand him. Hasan returns to Longharvest Lane again and notices the name "Hillinghead" and the tattoo symbol from the body's wrist scratched on the same wall.

Whiteman and Esther are followed by a man who later attempts to kill him in an air raid shelter. He is injured but kills his assailant and discovers a warrant card showing him to be a fellow police officer. 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 and informs her boss, DCI Barber. They discover further links between Mannix’s foster parents, the Morleys, and the Harker family. Hasan breaks into Harker’s abandoned home, where she is met by Andrew Morley, who has a collection of Defoe's dead bodies and talks cryptically about being part of a large group on an inevitable path, whose roles are to change the world and future. Maplewood's name is mentioned on an old phonograph recording. Morley then locks Hasan in an underground room.

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

Elias Mannix’s fingerprints are a direct match to Julian Harker’s, taken by Hillinghead in 1890. Hasan tracks down Elias’ birthmother, Sarah Mannix. Through Sarah, she discovers that DCI Barber is Harker's great-grandson, and Elias' father, but Barber disappears after taking Elias from HM Prison Feltham. Hasan goes to Barber’s home where she finds a 1941 recording addressed to her from Harker, confirming the teenaged Elias' statement that she will help him bomb London.

Whiteman is taken to the station by Hayden, who reveals he is part of the plot and tries to recruit Whiteman. A now elderly Elias Mannix/Julian Harker makes recordings addressed to others. Whiteman kills Hayden and goes to Harker House, killing Polly and Julian/Elias in revenge for Esther’s death. He is arrested, convicted, and sentenced to death by hanging.

Hasan informs SO15 about the recording and a key Barber left her, as the bomb is due to explode that day. DI Bothroyd accompanies her to Harker Bank where the key opens a vault, in Hasan’s name since 1941, containing a nuclear bomb. Hasan warns her family to leave London. She meets Barber and Elias, who holds the detonator. Barber plays a recording from 1941 where the elder Mannix encourages Elias to activate the bomb, saying the future it causes will be one where he feels truly loved. Bothroyd kills Barber, and Elias gives Hasan the detonator. She 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 1890, Mannix (now Harker) awaits the arrival of Defoe’s body, and sees Ashe photographing it moments after it appears. He hides in the alcove and is caught in the reflection of the window. He becomes closely acquainted with Hillinghead’s daughter Polly, a skilled pianist, and employs her as a caregiver for his mother. Harker purchases the bank and oversees construction of the bomb vault. After Hillinghead is arrested, Harker has him killed and tells Polly it was suicide. Harker and Polly happily marry and she gives birth to their son, Hayden. Shortly after the birth, Harker introduces her to the cult and confesses his true identity. Their close and loving relationship leads her to becoming a full and willing participant.

The now elderly Mannix and Polly prepare to poison Esther, and Mannix completes the series of recordings to instruct his cult.

In 1941, a regretful Harker makes a secret last recording, where he tells younger Elias all the previous recordings were lies, and voices regrets for all the lies as he did not end up finding the love that he so desperately sought. After Whiteman kills Polly, Harker convinces him to get the recording to Hasan, before he is also killed. Hayden Harker survives in this loop and leads a manhunt for Whiteman. Whiteman carves his name into the Longharvest Lane wall and hides the recording in a framed picture of his unit hanging in the local pub. He is then shot dead by Hayden’s officers.

Hasan discovers Whiteman’s carving in the wall in 2053, which directs her to the pub. She uses the Throat to travel back to 2023 and find the recording left by Whiteman. She then visits Sarah Mannix and calls her younger self, informing her of the bomb's second trigger. Older Hasan then plays Harker's new recording for Elias to hear. In response, he tears up the trigger number and doesn't detonate the bomb. Sarah embraces Elias, who disappears from existence along with older Hasan. As a result, Defoe's body never shows up in 1890, 1941, or 2023, so the history rewrites and the events from that point onward leading to the catastrophes experienced by Hillinghead, Whiteman, and Hasan never taking place.