Sam Hunter is concluding a mission in Tangier when an attempt on her life leaves her badly injured. She then disappears to a remote location to recover, regroup and retrain. She returns to work unannounced nearly a year later and manages to surprise her co-workers, who are suspicious about her disappearance. The team comes up with a plan to have Sam infiltrate a family headed by powerful millionaire Jack Turner.

Having been rought into the household by Turner's son Stephen to tutor his son Edward, Sam and the team continue to secretly take a look at Jack Turner's activities. Sam soon finds out that he is bidding on the contract to build a multi-billion dollar dam in Pakistan. Zoe Morgan and Ian Fowkes attempt to track a briefcase that was delivered to Turner.

The family make their way to a country estate, where Jack Turner and other interested parties bid on the dam contract. Sam attempts to to discover the amount of Turner's offer and runs into a man from her past, whom she believes could have something to do with the attempt on her life.

Jack Turner finds himself being forced to bid more than he can afford. Sam aims to find out how he will raise the money in a short time. It looks like Turner has hatched a diabolical plan with hedge fund manager Lewis Conroy and Sam looks for the connection. Aidan finds out some of Sam's secrets and begins an investigation of his own.

When Sam wakes up, she sees a familiar face in her hospital room, who promises to be an ally. He alludes to a connection from her past and refuses to give her all the information, but does tell her a secret that leads her to take action.

While Sam and Stephen grow closer, the Byzantium team is perplexed by Jack Turner's connection to Fatima Zahir (Meera Syal), a Pakistani presidential candidate. As Byzantium races against time, it seems inevitable that Jack will be successful. Meanwhile Sam's investigations into Hourglass lead her to connect someone from her previous mission with the Pakistani dam - helping her realize who their client is.

Determined to delve further into Hourglass, Sam makes a startling revelation relating to her past. However, her investigations trigger the interest of Byzantium's client. When Aidan instructs Natalie to uncover the leak in MI6, her shocking discovery exposes her to dangers on all sides. Byzantium discovers potentially devastating evidence regarding the dam, which Jack Turner plans to use to secure the bid.

Byzantium is instructed to eliminate Jack Turner, but after the attempted sleepytime fails, Sam becomes exposed as a spy, and is trapped and unable to escape from the house. Turner shows the evidence he has against Byzantium's client. Sam is secretly being poisoned to passing and finally starts to remember terrible memories that explain why the conspirators from Hourglass want her dead.