Mae Holland is a young woman a few years out of college who has just landed a job at the Circle, a major tech company voted “most innovative” four years in a row. The company has swallowed up its competitors with the invention of TruYou, a system that funnels a user’s accounts and online activity into one personal, traceable account. The Circle’s founder, Ty Gospodinov, invented the system and joined with two others, Eamon Bailey and Tom Stenton, to establish the company, which now wields an inordinate amount of power across the world.

Mae’s friend Annie Allerton got Mae the job at the Circle. Mae and Annie became close friends in college when Annie took care of Mae after she broke her jaw. While Mae is more reserved and self-conscious, Annie is vibrant, cheerful, and friendly. Annie’s quickly risen in the ranks of the Circle to become one of the Gang of 40, the top inner sanctum of the Circle. During Mae’s first days on the job, she’s quickly submersed in a fast-moving, hyper-connected world, a far cry from her job at the utility company she had back home. Mae struggles to keep up with the Circle’s mission of “Passion, Participation, and Transparency,” making a few faux pas on the job, such as not responding to a co-worker’s invite to brunch and not attending enough parties on the Circle’s campus, the series of offices, dorms, eateries, and shops that make up the Circle’s compound.

Back home, Mae learns that her father has multiple sclerosis, and her ex-boyfriend, Mercer, still visits them to offer help. The fact that Mae can’t help her parents, who are drowning trying to navigate health care red tape, makes her feel powerless. Mercer, who is staunchly against the invasive reach of technology, is also a thorn in Mae’s side. Whenever she’s home, he volleys her with attacks on her job, trying to make her question the Circle’s agenda. For Mae, her job is a golden opportunity and made her into an adult. She’s now an influencer whose tastes, opinions, and clothing can shape the market, and beyond that, she’s her parents’ savior when she’s able to put them on her company's health insurance plan and save them from their health woes. Mae, although annoyed and resentful, is resistant to Mercer’s attacks.

The Circle’s agenda grows towards something called Completion, something people hear being discussed on the Circle’s campus, but don’t know much about. Mae begins a relationship with two men on the campus: Francis, a man with a tragic past who is developing a child tracking software that puts chips in children’s bones, and Kalden, a mysterious man who doesn’t come up in any of the Circle’s directories and whom Annie suspects is a spy. Mae doesn’t feel satisfied with Francis, but he’s available. Kalden, on the other hand, is not available, as he only appears in Mae’s life sporadically.

Mae often goes kayaking to relieve her stress. One night, she’s caught “stealing” a kayak by two of the Circle’s SeeChange cameras, which are tiny, popsicle-sized surveillance cameras being posted across the world and promoted by the Circle. The next day at work, Mae is called into Bailey’s office and convinced she should go transparent and wear a camera 24/7 as a show of good faith. Mae complies. The Circle also convinces dozens of Congress members to go to raise the public’s faith in its leaders.

After going transparent, Mae’s popularity and visibility soar, which makes Annie jealous. Annie, who is participating in the Circle’s PastPerfect program, which traces one’s ancestry online, grows wearier of the Circle’s ever-invasive policies and programs after she discovers unsettling information about her family’s past. Mae is emboldened by her newfound power, however, which increases even more after she helps create Demoxie, a system to track public voting through Circle accounts, during a company meeting. Mercer, who has warned Mae of the Circle’s totalitarian grip, writes Mae a letter denouncing the Circle’s programs, telling her he’s going off-grid. During a demonstration of a new technology called SoulSearch that crowdsources live tracking of criminals, Mae decides to use the software on Mercer, to try to win him over to the power of tech. When Mercer releases he’s being chased by drones, he drives off a cliff and ends his life, while thousands of people watch online.

Annie suffers a nervous breakdown after Mae, who has been completely indoctrinated into the so-called benevolence and power of the Circle’s transparency agenda, enlists the public’s help in a show of sympathy for Annie and her misgivings over her family’s past.  In Book III, the last section of the novel, Mae sits beside Annie’s hospital bed. Earlier, Kalden revealed himself to be Ty, the founder of the company, and had asked for Mae’s help to stop the Circle from reaching Completion, but he has failed. The novel ends with Mae sitting contently beside a broken Annie, thinking about how Mae narrowly averted the downfall of the Circle by telling Bailey and Stenton about Ty’s plans.