After making a serious mistake during a live training exercise at Stansted Airport, MI5 agent River Cartwright is demoted to dead end work at Slough House in lieu of being fired. He is assigned by his superior Jackson Lamb to carry out surveillance on Robert Hobden, a journalist associated with far-right politicians Peter Judd MP and Roger Simmonds. His colleague Sidonie 'Sid' Baker has also been assigned to Hobden, discreetly copying the contents of his USB flash drive in a cafe. River is then instructed to deliver it to the service's Regent's Park Headquarters, much to his surprise.

Hassan's kidnappers, Moe, Larry, Curly and Zeppo, hold him hostage in a London terrace basement, with it transpiring that Simmonds is behind the kidnapping. Cartwright, having copied the contents of Hobden's USB drive before delivering it to Regent's Park, realises it contains no information and manipulates a rival, James 'Spider' Webb, into confirming the same. Standish begins having flashbacks to the death of a former colleague of hers and Lamb's, Charles Partner. Slough House's cyber expert Roddy Ho discovers he cannot hack Hobden's laptop as it is air gapped, furthering everybody's suspicions.

Sid, alive but in critical condition, is taken to hospital by MI5 security operatives who detain Cartwright. Lamb breaks him out, and the two deduce Moody was the gunman sent to kill Hobden. Moody returns to Slough House and attacks Min and Louisa when they re-enter, but is killed when he breaks his neck falling down the stairs in a scuffle. Lamb theorises that Slough House is going to be blamed for whatever operation Regent's Park is running and be disavowed if it fails.

Taverner meets with Lamb and reveals Ahmed is the nephew of the second in command of Pakistani Military Intelligence. His kidnapping and intended rescue is an unsanctioned false flag operation masterminded by herself to potentially increase cooperation with Pakistan as well as neutralise and intimidate the far-right. She claims to have an operative embedded with the Sons of Albion who organised the kidnapping, but will also facilitate the rescue. It is also revealed that Standish narrowly escaped a treason charge through her links with Partner and his death.

It is revealed that Moe (real name Alan Black) was Taverner's mole, but was found out and murdered by Curly, who then fled with Zeppo, Larry and Hassan. Driving out of London, Zeppo and Larry reveal that they never intended to go through with killing Ahmed, and entertain the idea of releasing him, becoming increasingly concerned with Curly's erratic behaviour and rhetoric.

Lamb and his team flee the property before "The Dogs" arrive, knowing that since the operation has not gone to plan that Slough House will be blamed. Cartwright is able to rescue Roddy Ho before he can be taken into custody, but Min and Louisa are unable to stop their colleague Struan from being taken from his home by operatives. Taverner calls Standish several times to flip her allegiance, alleging that Lamb was involved with Charles Partner's death, but she remains loyal to him when he comes to extract her. Leaving her house the two are apprehended by James Webb and head of security Nick Duffy, who force them into a car after searching Lamb. En route to Regent's Park, Standish is able to free herself and Lamb after pulling a Glock 17 from her handbag, the weapon having been taken from Moody and placed there by Lamb.

The rest of the team reconvene, and River recalls he took a photo of Taverner covertly meeting with Alan Black whilst carrying out surveillance training. Lamb "returns" Duffy's car to Regent's Park Headquarters and demands to speak with Taverner. She offers him a way out that will result in him being fired but not charged, but also threatens Standish and tells him Sid has died in hospital. However, Lamb informs her there is a bomb in the car he returned, which allows Cartwright to sneak in whilst Duffy's security team investigates.

He confronts Webb, alleging Taverner had him purposefully feed him incorrect intel during the Stansted exercise so he would fail and be demoted, as he had the photograph of her meeting with Black. After locating the photo and evading security, he and Lamb threaten to reveal Taverner's operation to the Director-General and the press.

Taverner confronts Judd at his home, revealing that calls he made to his far-right contacts after Hobden visited resulted in the mission being compromised and Black's death. Tierney returns from the US, ordering all loose ends be tied up after Taverner further lies to her about the operation. Curly forces Larry and Hassan to a secluded woodland at gunpoint to carry out the beheading. However, Larry frees Ahmed and attacks Curly with an axe, but flees which enables an injured Curly to recapture Hassan. Larry arrives at a seaport in Harwich, but is shot dead by "The Dogs" on the orders of Tierney, who plant a weapon on his corpse to justify the killing.

Curly prepares to kill Hassan outside what he believes is a Norman castle, which Ahmed points out is actually a folly. Lamb, Cartwright, Min and Louisa, with assistance from Roddy, are able to track them to the same location. Curly and River engage in a brief shootout, before he is knocked out by Hassan with a rock. "The Dogs" arrive, but the group prevent them from killing the detained Curly.