Set in the summer of 1953, the novel follows Andy Mills, a gay private detective who operates out of an office above the Ruby, a queer bar in San Francisco. Andy has settled into a comfortable life with his boyfriend Gene, the bar's manager, and his closest friends: Lee, a drag performer, and Elsie, the bar's owner.
Andy's routine breaks when Myrtle Bolton, visibly uncomfortable in the queer nightclub, hires him to find three missing members of her Mattachine Society discussion group. The Mattachine Society is a secretive organization pursuing equal rights for homosexuals that recently split: At two May conferences, members voted to oust its communist-affiliated founders, who believed in a distinct queer culture, in favor of an assimilationist approach focused on conformity and respectability. The three missing members are Edward, who vocally opposed the founders; his boyfriend Hank; and Daphne, a shy woman Myrtle clearly has feelings for. Myrtle provides physical descriptions but no last names, photos, or addresses, and suspects the ousted founders may be retaliating from their new base in Los Angeles.
Andy attends a Mattachine meeting, where the membership pledge briefly moves him with its promise that no queer person should face isolation alone. A guest psychiatrist's lecture dismissing gender-nonconforming people sours his goodwill. During the potluck, he learns Edward and Hank frequented a bar called the Silver Jay, where the bartender reveals that a gay motorcycle gang from LA visited weeks earlier and Hank left with them voluntarily. Andy drives south, promising Gene he will return for his birthday party.
Los Angeles stirs uneasy nostalgia; Andy grew up there but has not visited in seven years. He contacts Samuel Blanchard, Myrtle's LA connection, and attends a Mattachine gathering at Samuel's apartment. Samuel's partner Leo, who arranges fake marriages for gay Hollywood stars, reveals that Edward once worked as an informant for Mannix, a powerful studio fixer. A pharmacist named Vera provides the address for the ONE Institute, where the ousted founders now publish a magazine. The group dismisses the idea that the founders would target anyone.
At the ONE Institute, Mattie, a young Black woman helping produce the magazine, confirms no connection to the disappearances and warns Andy about Vera. At the Dahlia Pharmacy, Vera gives Andy a motorcycle club member's address in Long Beach in exchange for a favor. There, Andy finds the Bacchanals, a secret gay motorcycle club whose members are all military veterans. The club confirms Hank was prospecting to join but vanished two weeks ago along with Tommy, the sergeant at arms and the boyfriend of Russ, the club's president. They provide Hank's full name, Henry Nolton Jr., and suggest he returned to his wealthy family.
Andy stakes out the Nolton mansion in San Marino and follows a gold coupe to a building in Pasadena. Inside, he discovers it is the North Private Clinic, a psychiatric facility, and his mother Mary, a nurse there, emerges and recognizes him. They have not seen each other in seven years, and she insists he stay at the family home.
The Bacchanals ambush Andy to deliver urgent news: Tommy's body was found in an alley next to an empty needle, ruled a morphine overdose. Russ insists Tommy had been clean and hires Andy to prove it was murder. Andy also meets Will, a club member and police officer, the first other gay cop Andy has ever encountered. Will's openness about his identity stirs an attraction Andy does not act on.
At his mother's home, Andy navigates their strained relationship. When Mary overhears him say "I love you" on the phone to Gene, she assumes Gene is a woman. Andy fabricates a painful lie: Gene is a trauma nurse.
The case converges when Mary arranges a tour of her workplace. Dr. North, the chief psychiatrist, reveals that the clinic treats what he calls deviant sexuality. Andy realizes it is a conversion therapy facility. He watches Dr. North inject an elderly patient with apomorphine, a drug that induces violent vomiting, while projecting images of men on a screen. A nurse confirms that Hank is a patient on the locked floor, committed by his family against his will.
Andy confronts Edward at the Nolton estate. Edward admits he could ask the family to release Hank but wants time apart, framing the commitment as justified for leaving their relationship. Andy observes Edward dining warmly with Hank's parents and sister Ginny while Hank remains institutionalized.
The investigation tightens. Andy discovers Vera runs an illegal drug operation through forged prescriptions, and Russ's system for keeping Tommy clean involved low-dose morphine pills obtained from her. Vera reveals that Dr. North was formerly a studio doctor and confirms that apomorphine in large doses can kill, potentially being mistaken for a morphine overdose if no blood testing is performed, which the coroner neglected.
Andy reconstructs the full picture: Edward, bitter over Hank leaving him, contacted Hank's parents and enlisted Dr. North to commit Hank. Edward had leverage from his days as Mannix's spy, having learned that North impregnated a young actress, botched her abortion, and she died. Edward and North kidnapped Hank from the Bacchanals' garage, and when Tommy tried to intervene, Edward injected him with a lethal dose of apomorphine, staging the death as a morphine overdose.
Andy assembles an unlikely coalition: the Bacchanals, Samuel and Leo with a studio van, Mattie from ONE, and Vera to provide medical care. At the clinic, Andy convinces the night staff, including his mother, to gather in the lobby by fabricating a threat, while Will in police uniform lends authority. The Bacchanals help willing patients escape.
Russ deviates from the plan and sets the building on fire. Andy and his mother rush through the burning wing, confirming all rooms are empty. Outside, Andy tells his mother he lied about Gene. He climbs onto Will's motorcycle, and as they pull away, he sees understanding cross her face as she watches her son ride off with his arms around another man, the burning clinic behind her.
Hank confirms the full story: Edward and North kidnapped him, and Edward killed Tommy when he tried to help. Russ, hearing proof his boyfriend was murdered saving Hank, breaks down.
Back in San Francisco, Andy wraps up the case. Lee located Daphne in Sea Cliff; she left the Mattachine voluntarily, choosing to marry a male friend rather than continue a struggle she believes will not succeed in her lifetime. She asks Andy to tell Myrtle she truly cares for her. Myrtle absorbs the heartbreak with composure and pays Andy.
Russ reports that no charges are being filed and the fire investigation has stalled. Edward has become engaged to Hank's sister Ginny, securing his place in the wealthy family now that Hank has been cast out. Hank is staying with the Bacchanals.
At the birthday party, Elsie reveals that every guest is someone Andy has personally helped and suggests that Andy has finally begun to find himself worth looking at. On his actual birthday morning, Andy sits at his desk waiting for his mother's annual phone call. At noon, he accepts she is not going to call and begins to grieve. Then the phone rings.