Сам Хънтър – агент в елитната частна разузнавателна фирма „Бизентиъм“, приключва мисия в Танжер, когато се опитват да я убият. Без да знае кой седи зад покушението, тя изчезва, без да оставя следи, докато се възстанови от раните си и възвърне самообладанието си. Година по-късно Сам се връща на работа, с което изненадва колегите си, които остават усъмнени в отсъствието й. Сред тях е и Ейдън Марш – тайният ѝ любовник. Шефовете ѝ Рупърт Кийл и Дийкън Крейн също не са сигурни в нея, но имат нужда от способностите ѝ за нова мисия. Екипът прави план, според който Сам трябва да се внедри в семейството на милионера Джак Търнър – човек с криминално минало, чиито амбиции са в разрез с тези на клиент на „Бизентиъм“.

Сам успява да влезе в семейството на Търнър като гувернантка на детето на сина му Стивън. Така екипът й продължава тайно да документира активностите на Джак. Те знаят, че той участва в мултимиларден търг за язовир в Пакистан, но истинските му мотиви не са им известни. Междувременно членовете на екипа Зои Морган и Йън Фоукс проследяват куфарче, доставено до Търнър.

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.

A mysterious assassin tells Sam that he is trying to protect her from Hourglass, a conspiracy orchestrated by five multinational corporations. Sam's investigation of the conspirators takes her to a country estate where she had been years before - it was where she was taken as a child after her mother was murdered and she was kidnapped.

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 ordered to kill Jack Turner, but when the assassination attempt fails, Sam is exposed as a spy, and trapped inside the house. Turner produces the evidence he has against Byzantium's client, while Sam - secretly being poisoned to death - at last begins to recover terrible memories that explain why the Hourglass conspirators want her dead.