Best friends Debbie Claybourne and Judith Burton have their world's turned upside down when Debbie discovers Judith isn't a forensic accountant, she's an international assassin! When a mysterious figure from Judith's past emerges and a hit goes badly wrong, they're forced to go on the run together.

In the aftermath of the Gala, Judith scrambles to uncover who set her up. Debbie searches for answers about her husband and makes a terrible discovery. The Director gives Judith one last chance to finish her assignment, and Sam discovers where Billy Donovan is hiding.

Debbie and Judith's road trip to Europe takes an unexpected turn when they're forced to travel by luxury train, only to discover they've been followed. Sam's one night stand is not what she seems. Queenie agrees to help Sam find a body. The Albanians are given a lead. Debbie makes an impossible discovery.

Judith and Debbie make it to Spain, reeling from their discovery about David. The Director gives Judith an impossible choice. Interpol detective Jacques Boucher investigates a body on the train. Debbie and Judith work together to find Billy Donovan, and Debbie gets her groove back. Sam and Queenie make a dangerous decision. The Albanians close in.

An unlikely trio sets off for Monte Carlo to track down David's missing bribe money. Judith is forced to adopt an old cover, Debbie comes up with a very risky plan and neither of them know if they can trust Billy Donovan. Sam disappoints Queenie. Mrs. Williams is visited by a former colleague.

With Judith in life-threatening peril, Debbie and Billy must decide what to do next. A lifeline from Sam provides Judith with terrifying information, and secrets about her past are revealed. Queenie confronts The Director, Jacques offers Debbie a deal, and Sam and Queenie take matters into their own hands and embark on a dangerous mission.

All roads converge in Geneva as everyone closes in on the stolen money. Judith discovers some unexpected guests, Interpol and the Albanians make their move, and romance blossoms. When the plan goes sideways, Judith and Debbie find themselves in mortal danger.

Back in London, Judith and Debbie's friendship seems beyond repair. Queenie learns a shocking truth, Sam deals with the fallout of his actions, and the Director takes Judith back into the fold. As the final piece of Ana's plan falls into place, Debbie and Judith hurtle towards a spectacular season finale.