In a futuristic dystopian society, citizens, known as "members", live under the Collective, the legislative body which monitors and controls the people's actions. Silas Nicholas Hoult, a citizen who works as an illustrator for Atmos, returns home one night to find two citizens detained by officials, a reminder of a purported epidemic called Switched-On Syndrome (SOS), a multi-stage disease that supposedly restores human emotions. Sufferers who do not take their own lives progress to stage four and are detained in the Defective Emotional Neuropathy Facility (the DEN), the Collective's dreaded institution from which no one ever leaves. The next day at work, a suicidal employee jumps to his death, and the emotionless workers coldly analyze the moment. Silas is the only member of his group who notices fellow worker Nia show an emotional reaction, and later in a team meeting he catches her expression again, hinting at something forbidden and dangerous beneath the surface of their antiseptic world.
The following day, Silas grows distracted during a conference at Atmos, begins to fall asleep more often, and experiences a nightmare for the first time. He goes for a check-up and is befriended by an official named Jonas with stage 2 SOS. Silas is diagnosed with Stage 1 and given a prescription, but his condition worsens as his drawings become more expressive and his interest in Nia deepens. Suspecting that Nia may also have SOS, he confronts her, but she insists she is clean. One night, he follows her into the bathroom and they touch for the first time. She reveals that she has had SOS for over a year, hiding it to avoid discovery and ostracisation. They kiss and begin to meet on subsequent nights, exploring and sharing their emotions in secret. One night, in the bathroom, they hear Leonard, the company manager, and Silas converses with him. Leonard spots Nia's workstation powered on and reveals that he has been monitoring Silas. Silas resolves to find a job away from Nia, moving into a gardening section.
Leonard introduces Silas's replacement, Dominic, to Nia the next day. During lunch, she suffers a slight anxiety attack. At night, while collecting prescriptions, Jonas invites Silas for a walk and discloses that he is part of a secret support group and offers help. He decides to attend the clandestine meetings, where he meets fellow members Bess, Peter, Thomas, Gil, Max and Alice, and learns that the DEN's patients often end their lives—half the time—through encouragement from others. Nia then shows up at his apartment, and they have sex, agreeing to spend more time together. Soon after, the Ashby ENI cure for SOS is announced and successfully developed. Fearful, they decide to flee to the Peninsula, a secluded, primitive region, while their support group warns that they may never return if they succeed. Jonas instructs Silas to ask for Oliver, a pilot, to fly him to the Peninsula. The couple plans to head toward Wellington, the nearest border settlement, on Saturday. However, Nia becomes pregnant after a conception summons and is taken to the clinic, from which she is then sent to the DEN.
In a near-panic Silas seeks out Jonas, who urges calm and tells him to return home. Silas sinks into depression as he awaits what fate may bring. Bess, having learned from Jonas and seeing that Nia is Stage 4, takes her to a room where Jonas and Gilead brief Eva, a deceased Stage 3 SOS patient, and help Nia fake her death by swapping identity implants so Eva is considered alive and Nia dead. The ruse succeeds and Nia leaves the DEN but does not find Silas at his apartment. Meanwhile, Silas learns that Bess, Jonas and Gilead were betrayed by Max and given the cure. He goes to the DEN, where they tell him Nia died, and he contemplates suicide on a rooftop, but instead receives the cure himself. Returning to his apartment, he finds Nia alive, and discovers that his SOS treatment was in vain. They have only about five hours left before Silas’s emotions are erased by the cure. Nia urges him to resist by remembering the moment they touched, and she makes him promise not to give up on her.
The next morning, an emotionless Silas remembers her and the love they shared, though his recollection is limited and the feelings may fade. They nonetheless set their plan in motion and board the Wellington train, heading toward the unknown Peninsula. Silas watches Nia from a distance and, as they travel, seems affected by her depiction of emotions in the early moments of their connection. At some point, he moves closer and sits beside her, reaching to hold her hand, and she responds by gripping his fingers tightly—an imperfect, fragile glimmer of the emotions they once had, and perhaps a prelude to a fragile new bond that could defy the system that wants to erase their humanity. The journey leaves them poised at the edge of a broader, uncertain future, where memory, feeling, and connection clash with the world’s strict rules, and where love itself tests the price of freedom.
