Tessa sees her sister Marian appear on a BBC broadcast, identified as a suspect in an IRA armed robbery.

Tessa meets DI Fenton at the police station and lies, claiming Marian is pregnant, to help portray her as a victim.

DI Fenton reveals a pipe bomb was found at St. George's Market on the exact day Marian visited with Tessa and Finn.

Tessa realizes Marian deliberately brought her and baby Finn to the market to use them as innocent cover for the bomb.

After weeks missing, Marian reappears at the lough and confronts Tessa about contacting the police.

Marian confesses she has been an IRA member for seven years and asks Tessa to relay messages to her MI5 handler in her place.

Tom refuses to give consent for Tessa to take Finn out of Northern Ireland, leaving her trapped.

Tessa meets MI5 agent Eamonn Byrne at the lough and agrees to become an informer, learning the Visa card communication system.

At cousin Aoife's wedding, Tessa meets IRA unit leader Seamus for the first time and he takes an unsettling interest in Finn.

Eamonn warns Tessa that refusing Seamus's invitation to join the IRA as a scout will cast suspicion on Marian and endanger her life.

Tessa sits outside a police station in Saintfield for hours, recording every car that enters and leaves for Seamus.

Tessa and Damian travel to Mallow posing as a couple on holiday, and Damian attempts to shoot Lord Maitland but misses.

News of the failed assassination attempt spreads, and the IRA publicly calls a cease-fire.

Two masked men kidnap Tessa and drive her to a remote farmhouse where Seamus is waiting with plastic sheeting already laid for their bodies.

Marian kills Seamus with her hair clasp; the sisters convince the remaining men to let them flee, escape across the border, and are given new identities.

Northern Spy opens with BBC editor Tessa Daly discovering her sister Marian is linked to an IRA armed robbery, and as incriminating evidence mounts, she is forced to accept the unthinkable. When Marian finally reappears, she confesses to seven years of IRA membership while also revealing she has become an MI5 informer — and she needs Tessa to take over contact with her handler. Trapped by her ex-husband's refusal to let baby Finn leave Northern Ireland, Tessa reluctantly becomes an informer herself, meeting MI5 agent Eamonn Byrne at the lough. Her involvement escalates when IRA unit leader Seamus recruits her directly, sending her on increasingly dangerous scouting missions and ultimately into an assassination operation that fails by a single missed shot. After the IRA calls a cease-fire, Seamus kidnaps both sisters and brings them to a remote farmhouse to be executed; Marian kills him to save them, and the two escape across the border to begin new lives in Ireland under assumed identities.

The headline story of two sisters caught between the IRA and MI5.

Tessa walks with baby Finn near home and reflects on how Northern Ireland's conflict remains unresolved despite the Good Friday Agreement.

Tessa drives to Templepatrick and questions an employee at the service station where Marian's armed robbery took place.

DI Fenton questions Tessa about Marian's suspicious paramedic choices at a bombing scene and a hidden burner phone found in her apartment.

Tessa's mother returns from visiting Eoin Royce, a former IRA member, who confirms the IRA now forces ordinary civilians to commit crimes for them.

Tessa's mother is effectively pushed out of her cleaning job with the Dunlop family because of Marian's suspected IRA connection.

Tessa discovers an empty weapons cache in the fields directly behind her home.

Detective Sergeant Cairn plays Tessa a recording of Marian confirming a gun importation from Serbia with IRA figure Cillian Burke.

Tessa goes to an IRA-sympathizer bar and demands to know where Marian is; the bouncers tell her Marian will call when she wants.

Belfast is placed on high alert for a large-scale IRA attack; Tessa dives to the floor of a chemist's shop when wooden pallets fall from a lorry outside.

Marian finds Tessa on the top deck of a bus and passes along intelligence about an IRA plan to blackmail a financier close to the prime minister.

Tessa's anger softens enough that she fetches Finn from Tom and drives to let Marian hold and embrace her nephew for the first time.

During Aoife's wedding reception, Marian plants a listening device behind the eye of a mounted stag's head at the IRA-owned Balfour Hotel.

Tessa meets Seamus, Damian, and Niall in the back room of Gallagher's Pub, answers Seamus's pointed questions, and is accepted as an IRA scout.

Tessa procures two gallons of kerosene for Damian and helps Niall clean the back room of the pub.

The sisters are resettled in a safehouse in Dalkey, Ireland; Tom is furious when Tessa tells him Finn is with her across the border.

Pool 2 traces the secondary events and human consequences that give Northern Spy its texture and weight. From the opening chapters — where Tessa investigates the robbery site herself and her mother desperately seeks advice from a former IRA member — through the mounting evidence that Marian is genuinely a terrorist, the narrative builds an atmosphere of pervasive fear and loss. The Daly family absorbs quiet but devastating collateral damage: Tessa's mother loses the long-term employment she depends on, a false alarm in a chemist's shop exposes a city conditioned to expect bombs, and Tessa must eventually relent and allow her estranged sister to hold baby Finn. As Tessa becomes more embedded in the IRA unit, she plants a listening device at a wedding reception and is formally accepted into the organization before carrying out errands she knows will fund violence. The pool closes with the sisters' narrow survival and forced relocation to a safehouse in Dalkey, and the fury of Finn's father when he learns where his son has been taken.

The everyday dangers and collateral costs rippling through the Daly family.

Tessa's BBC editorial team debates Cillian Burke's likely acquittal and the justice minister's controversial call for citizens to spy on their neighbors.

DS Cairn tells Tessa that Marian secretly flew to Belgrade for four days with other IRA members, destroying Tessa's last hope of her innocence.

Tessa decides to cooperate with Marian, concluding that the only way Finn can truly be safe is for the conflict itself to stop.

DI Fenton visits Tessa at her BBC office and questions her about explosives, including whether she knows what nitrobenzene smells like.

Marian tells Tessa that the IRA is importing a 45-ton gelignite shipment and that she scouted a landing site on the north coast for it.

Tessa receives a burner phone from the IRA and receives her first direct orders from Seamus assigning her to scout in Saintfield.

Tessa hands her scouting notes to Seamus at a pub meeting; Niall immediately proposes planning a killing based on one of the cars in her report.

Eamonn tells Tessa he suspects Marian may be feeding MI5 disinformation, citing a drone sweep of the Armagh arms drop that found nothing.

Tessa confronts Marian about the disinformation allegation; Marian reveals that Tessa's miscarriage was one of the moments that made her decide to defect.

Seamus reveals that Tessa's BBC colleague Colette has been secretly feeding the IRA information about every important guest in the studio.

A last-minute hurricane forecast forces Seamus to abandon the bombing plan and order Damian to use a sniper rifle instead to kill Lord Maitland.

Eamonn confirms that a boat carrying the IRA's massive gelignite shipment was sunk, proving that Marian had been a genuine informer all along.

During Seamus's farmhouse interrogation, Marian confesses to being an informer to draw focus away from Tessa, and Tessa then confesses as well.

Tessa learns that MI5 deliberately chose not to rescue her and Marian from the farmhouse in order to protect Cillian Burke as a more valuable asset.

Cillian Burke is publicly revealed to have been an MI5 informant; years later, Tessa brings Finn to visit Dunseverick Castle in a Northern Ireland finally at peace.

Pool 3 tracks the pivotal revelations and turning-point decisions that drive Northern Spy's central tension from beginning to end. The story opens in an atmosphere of institutional suspicion — Tessa's news team debates whether Cillian Burke will ever face justice — before shattering into crisis when proof emerges that Marian secretly traveled to Belgrade with the IRA. Tessa's calculated decision to cooperate pulls her into a web of intelligence operations, where she learns of a massive arms shipment, passes critical information to MI5, and eventually uncovers a spy operating inside her own BBC workplace. The web tightens through a sequence of devastating turns: Eamonn's suspicion that Marian is feeding disinformation, Marian's raw confession about the personal grief that drove her to defect, and a last-minute operational change that puts Tessa inside an assassination. The pool closes with the two most consequential revelations of the novel — that MI5 was prepared to let both sisters die to protect a more valuable asset, and that the IRA's most prominent figure was himself a British informant all along.

The key discoveries and decisions that shift the story's direction at every turn.