Ava Faulkner (Jessica Chastain) is a recovering addict and former soldier turned assassin. In France, she kidnaps her new target, an English businessman. Before she kills him, she questions him on why someone wants him dead. Unbeknownst to her, another woman electronically eavesdrops on the conversation. Afterwards, Ava flies to Boston where she visits with her estranged sister Judy and her mother Bobbi who is hospitalized for angina pain. Ava has not seen them in eight years.

Ava's handler and former Army superior, Duke, sends her to Saudi Arabia to kill a German general. Ava lures the general into a trap and injects him with a poison to make it appear he died of a heart attack. She is interrupted by the general's security guards. A gunfight ensues, leaving all the men dead.

Ava escapes and travels to Barneville-Carteret where Duke apologizes for the botched operation, insisting that the bad intel was a simple mistake. He gives her time off to decompress and she returns to Boston and meets Michael, her former fiancé who is now in a relationship with her sister Judy.

In British Columbia, Duke meets with his superior, Simon. Simon's daughter Camille is the woman who had earlier eavesdropped on Ava's hit. Simon believes Ava is a liability and that her questioning of targets demonstrates insufficient commitment to their operation. After Duke leaves, Simon reaffirms the hit on Ava. She kills her attacker and then confronts Duke who insists that it was a random drug addict attack. That night, Ava goes to dinner with Judy and Michael but their conversation does not go well. The next morning, Judy meets Ava and tells her Michael is missing. Realizing he has started gambling again, Ava rescues him from a gambling den run by a woman, Toni, to whom Michael is indebted.

Duke revisits Simon and reveals he knew Ava was set up. A fight ensues between the two men, resulting in Simon killing Duke. He sends a video of Duke's death to Ava. A heartbroken Ava goes to Judy's house, where she invites Michael to run away with her but he declines, revealing that Judy is pregnant. Ava heads to Toni's den where she kills some of her men before giving Toni a bag of money to pay off Michael's debt. Ava starts to strangle Toni, and is about to break her neck, but then changes her mind, letting her alive while warning her to stay away from Michael.

Back at her hotel, Ava is attacked by Simon. They fight, with both sustaining serious injuries. Exhausted, Simon flees when the fire alarm goes off, warning Ava that he will kill her if he sees her again. Ava pursues Simon, cornering and killing him under the Zakim Bridge. Ava goes to her sister's house, warning Judy to leave the country and giving her the number to a Swiss bank account filled with Ava's earnings. Before she leaves, Michael gives her a letter from Duke, who says that he is happy with how his life turned out. As she walks down the street, Ava is stalked by Simon's daughter, Camille.