The novel opens during late summer at Goshen Beach, Rhode Island, where Danny Ryan, a longshoreman and low-level enforcer for the Irish Murphy crime family, watches an unknown woman emerge from the ocean. Danny sits on the sand with his wife, Terri, and friends including his best friend and brother-in-law Pat Murphy, Pat's younger brother Liam Murphy, and Jimmy MacNeese. They have rented cottages near the beach, as their families do every August, calling the area "Dogtown by the Sea" after their working-class Providence neighborhood.
Danny is restless. He once loved working as a swordfish fisherman but gave it up because Terri refused to live away from her family. He married into the Murphys and took a union dock job arranged by his father-in-law, John Murphy, the Irish mob boss in Dogtown. Danny does collections and occasional truck hijackings but has never received the advancement he expected. Peter Moretti, a rising captain in the Italian crime family led by the aging boss Pasco Ferri, has suggested that the Murphys keep Danny down because they fear the old Ryan dynasty being restored: Danny's father, Marty Ryan, once co-ran the operation alongside John before alcoholism destroyed him. The Irish and Italian families have been allies for a generation, having jointly fought off New York crime bosses and carved up Providence's rackets.
At Pasco's annual Labor Day clambake, Danny discovers that the woman from the beach is Pam Davies, the new girlfriend of Peter's brother Paulie Moretti. Also present is Cassie Murphy, Danny's sister-in-law, recently returned from rehab. The novel reveals in flashback that Pasco sexually assaulted Cassie when she was 14; her descent into substance abuse stemmed from that trauma.
Late that night, Pam accuses Liam of groping her on the beach. Pat apologizes, but hours later Peter, Paulie, and their enforcer Sal Antonucci beat Liam with a baseball bat, fracturing his skull and rupturing his spleen. Pasco intervenes, forbidding retaliation from either side.
As Liam recovers, Pam leaves Paulie and moves in with Liam. Peter sees opportunity in his brother's humiliation, using it as a pretext to seize the Murphy-controlled docks. When a minor Murphy associate makes a crude joke, Paulie shoots the man dead in front of Danny. At the Glocca Morra, the Murphy headquarters pub, John counsels restraint, arguing that a war would destroy the outgunned Irish. His accountant and strategist, Bernie Hughes, explains that Peter needs the docks to position himself as Pasco's eventual successor.
John secures a fragile peace through Pasco: The Murphys won't retaliate, but Liam must stop seeing Pam. Liam refuses. At the murdered associate's funeral wake, he arrives with Pam and announces they have eloped. John publicly accepts the marriage. Cassie observes and says quietly, "This is going to end badly."
Pasco arranges a formal sit-down. The meeting seems resolved until Paulie insults Pam and Liam challenges him to a fistfight. A sniper's bullet hits Paulie in the leg. Danny learns that Liam secretly arranged the shooting through Mickey Shields, an operative of the Irish Republican Army (IRA), promising the militant organization help with guns in exchange. The peace is destroyed.
Peter offers Danny a chance to sit out the coming war in exchange for betraying the Murphys. Danny is tempted but cannot abandon Pat. That night, Terri reveals she is pregnant. The Morettis launch coordinated strikes, killing three Murphy associates. Pat assumes command, and Danny volunteers to lure Steve Giordo, a professional hitman brought in by the Morettis, into an ambush by impersonating Liam and using disinformation he fed to Peter. The operation goes badly: Shields is killed, Danny is shot in the hip, and Jimmy barely gets him out alive.
Danny wakes in the hospital beside a red-haired woman: his mother, Madeleine, whom he has never known. Born into poverty in California, Madeleine reinvented herself, eventually building a fortune through strategic relationships with powerful men. She uses Justice Department connections to protect Danny and pays for his hip reconstruction and rehabilitation. FBI agent Phillip Jardine approaches Danny, offering the witness protection program or a role as informant. Danny refuses both.
Before Danny can flip Sal Antonucci against Peter, the Murphys car-bomb Sal's secret lover, Tony Romano, driving Sal back to the Moretti side. Consumed by grief, Sal runs Pat Murphy down with a stolen car and hides the corpse. John humbles himself by going to Sal to ask for the body's location. Danny and Jimmy recover Pat's remains from a shallow grave. Danny assumes leadership of the Irish operation because John is shattered by grief, Liam is hiding, and no one else remains.
Both sides exhaust themselves. The fighting pauses. In June, Terri gives birth to their son, Ian Patrick Ryan, and Danny experiences genuine happiness for the first time. When the Morettis resume warfare, Danny retaliates and brokers an alliance with Marvin Jones, a Black crime boss who wants the Morettis out of South Providence. Peter has Sal assassinate Marvin, but Liam independently tracks and kills Sal. Peter avoids acknowledging the Murphys' involvement, instead spreading a false account that Black gang members killed Sal in retaliation for Marvin's death.
Terri discovers a lump in her breast. The cancer is malignant, stage three, and by autumn it metastasizes to her liver. Doctors give her months.
Frankie Vecchio, a Moretti soldier, approaches the Murphys offering to betray a 40-kilo heroin shipment worth six million dollars. Danny distrusts the offer and opposes dealing drugs. Bernie speaks against it on moral and practical grounds. But Liam argues the score will fund their escape, and John overrules the objections. The hijacking succeeds: Danny's crew splits the haul, hiding 10 kilos at a cottage in Mashanuck Point while Liam takes 25 to the Glocca Morra.
Danny pieces together that the operation was a trap. Chris Palumbo, Peter's right-hand man, sent Vecchio to lure the Murphys into possessing heroin. Jardine, Chris's corrupt FBI partner, raids the Glocca Morra before Christmas, seizing 12 of the 22 kilos stored there while pocketing 10 for himself. John is arrested. Jardine then captures Liam at a motel after Pam, beaten and terrorized by Liam, calls in his location. Jardine drives Liam away, shoots him, and stages the death as a suicide.
At the hospital, Danny promises the dying Terri he will care for their son. He goes to the Mashanuck Point stash house and sets a countdown: If he does not check in within 15 minutes, Ned Egan, Marty's longtime bodyguard, and two young recruits will kill Chris Palumbo's family. When Chris arrives to claim the heroin, Danny holds him at gunpoint. Chris backs down. Danny then lures Jardine to the beach, offers a deal, and when Jardine draws his weapon, shoots him dead. Before the meeting, Danny threw all 10 kilos of heroin into the ocean, refusing to build a new life on that sin.
Moretti soldiers arrive at Marty's cottage seeking the drugs. Marty shoots one from under his blanket; Ned kills the other. Danny arrives, gathers his infant son and a reluctant Marty, and drives away from Dogtown for the last time. On the road, Marty sings "Leaving of Liverpool." Danny heads into the unknown with no money, no plan, and no resources, carrying only his father, his son, and the promise he made to his dying wife.