Caleb Smith, a programmer at the search engine company Blue Book, wins an office contest for a one-week visit to the luxurious, isolated home of the CEO, Nathan Bateman. Nathan lives there with an unspeaking servant named Kyoko, who, according to Nathan, does not understand English. Nathan reveals that he has built a humanoid robot named Ava with artificial intelligence. Ava has already passed a simple Turing test, and Nathan wants Caleb to judge whether Ava is genuinely capable of thought and consciousness as well as whether he can relate to Ava despite knowing she is artificial.
Ava has a robotic body with the physical form and face of a woman and is confined to her apartment. During their talks, Caleb grows close to her, and she expresses both a desire to experience the world outside and a romantic interest in him, which Caleb comes to return. She can trigger power outages that temporarily shut down the surveillance system that Nathan uses to monitor their interactions, allowing them to speak privately. The outages also trigger the building's security system, locking all the doors. During one outage, Ava tells Caleb that Nathan is a liar who cannot be trusted.
Caleb grows uncomfortable with Nathan's narcissism, excessive drinking, and crude behavior toward Kyoko and Ava. He learns that Nathan intends to upgrade Ava after Caleb's test, wiping her memory circuits and in effect "killing" her current personality in the process. After encouraging Nathan to drink until he passes out, Caleb steals his security card to access his room and computer. He alters some of Nathan's code and discovers footage of Nathan interacting with previous android models with the appearances of women who were held captive. Kyoko reveals to him that she is also an android by peeling off her skin. Later, Caleb examines himself and cuts open his own arm to determine if he himself is an android.
At their next meeting, Ava cuts the power. Caleb explains what Nathan is going to do to her, and Ava begs him for help. Caleb informs her of his plan: he will get Nathan drunk again and reprogram the security system to open the doors in a power failure instead of locking them. When Ava cuts the power, she and Caleb will leave together, locking Nathan in behind them. Ava later encounters Kyoko for the first time when Kyoko enters her room.
Nathan reveals to Caleb that he observed Caleb and Ava's secret conversation with a battery-powered security camera. He says Ava has only pretended to have feelings for Caleb, who was deliberately selected for his emotional profile so he would try to help her escape. Nathan says this was the real test all along, and that by manipulating Caleb successfully, Ava has demonstrated true consciousness. Moments later, when Ava cuts the power, Caleb reveals that he had suspected Nathan was watching them, so had already modified the security system when Nathan was previously passed out. After seeing Ava on the security cameras leave her confinement and converse with Kyoko, Nathan knocks Caleb unconscious and rushes to stop the two robots escaping.
Ava attacks Nathan but is overpowered and damaged before Kyoko and then Ava both stab and kill Nathan. In the process, Nathan disables Kyoko. Ava finds Caleb, and asks him to remain where he is while she repairs herself with parts from earlier androids, using their artificial skin to take on the full appearance of a woman. Instead of returning to Caleb however, Ava leaves the area using Nathan's ID card to unlock the glass security door, which locks behind her, leaving Caleb trapped inside. Ignoring Caleb's pleas, she glances briefly at the bodies of Nathan and Kyoko before leaving the facility. She then escapes to the outside world in the helicopter meant to take Caleb home. Arriving in a city, she blends into a crowd.
