The book follows the story of Mae Holland, a recent college graduate who lands a job at The Circle, a powerful technology company run by the so-called "Three Wise Men." Mae owes her job largely to her best friend and college roommate, Annie, one of the forty most influential people in the company. Mae starts out in Customer Experience (CE), the firm's customer service department, but quickly climbs the company ladder. From the beginning, Mae is impressed by amenities at the Circle, including access to top-notch technology, dorm-like housing, gyms and recreation activities and parties. Mae's very first day at the Circle ends with a party where she encounters Francis, who remains a love interest throughout the rest of the novel. Later, Mae encounters and quickly becomes romantically involved with a mysterious colleague named Kalden, even though she is unable to verify his status at the company, or even obtain his last name.
Meanwhile, the Circle continues to develop a range of sophisticated technologies, including SeeChange, light, portable cameras that can provide real-time video with minimal efforts. Eventually, SeeChange cameras are worn all day long by politicians wishing to be 'transparent', allowing the public to see what they are seeing at all times.
Another subplot concerns the diagnosis of Mae's father with multiple sclerosis, which eventually leads Mae to enroll her parents on her health care, which in turn drives them to install SeeChange cameras in their home. After a brief incident with her own legal issues, Mae ends up agreeing to wear a SeeChange device herself (called going transparent), epitomizing her own growing role in the company, epitomized by a public talk in which she insists, "Secrets are Lies", "sharing is caring," and "privacy is theft." Mae's job now mainly consists of touring the campus and showing customers future products; any hints of her own doubts about privacy seem dropped, in contrast to her ex-boyfriend Mercer, who eventually insists on going "off the grid" to escape the increasingly far-reaching influence of the Circle and its technologies. Kalden calls Mae, warning her that The Circle must be stopped, but she refuses to listen.
Meanwhile, Annie becomes envious of Mae's success and volunteers to be the test subject for PastPerfect, a new product that tracks a person's family history and activities, to regain her standing within the company. Disturbing facts about Annie's family history come to light and she becomes increasingly stressed, eventually causing her to fall into a catatonic state. Embarrassed both by her parents' distrust of SeeChange monitoring in their home and the conduct of her ex, Mercer - all of which have been publicly accessible to Mae's millions of followers - Mae becomes upset that they have embarrassed her in this way and, while demonstrating a program designed to catch fugitives within minutes, uses it to find Mercer who attempts to escape using his truck and ends up driving off a bridge and is killed after drones prevent his escape from being constantly watched. While initially depressed, Mae soon rationalizes Mercer's death with the help of charismatic 'Wise Man' Eamon Bailey, concluding that he was an extremely depressed, antisocial human being who refused society's help, comparing his actions to a person committing suicide by jumping out of the window when being visited by a doctor.
Kalden is, in fact, the elusive 'Wise Man' Ty Gospodinov, the original inventor and founder who came up with the idea for the company. Mae agrees to meet him in secret and he tells her that a totalitarian regime will soon arise if nothing is done to stop it, equating it to a surveillance society. He explains the need for privacy in the digital age and asks her to help him take down the Circle, saying he cannot do it alone. Mae thinks that he is crazy, but pretends to go along with him, but then betrays him by telling the other founders of the Circle what has happened, who ensure that Ty is silenced. The book ends with Mae looking at Annie in the hospital, wondering when the time will come that people's thoughts will be knowable and public information, saying that "the world deserves nothing less and will not wait".