The following version of this book was used to create the guide: Lehane, Dennis. Small Mercies. HarperCollins, 2023.

In the summer of 1974, a woman named Mary Pat Fennessy struggles to juggle her responsibilities as a single mother with the gathering unrest of the desegregation busing crisis in South Boston. Mary Pat's daughter, Jules, goes missing on the same night that a Black man named Auggie Williamson (whose mother, Calliope, works with Mary Pat) is found dead on the train tracks at Columbia Station. Mary Pat contacts her sister, Big Peg, whose daughter informs Mary Pat that Jules was last seen spending time with Rum Collins, George Dunbar, and Brenda Morello, the latter two of whom are known to be involved with Marty Butler's crew, a criminal organization that runs South Boston. When Mary Pat fails to get a straight answer about Jules' whereabouts out of either George or Rum, she gets in touch with an old friend, Brian Shea, who also works for Butler, and asks him to locate Jules, which he promises to do. Mary Pat visits her ex-husband, Ken, and insults him when she discovers that he has begun dating a Black woman. When Brian fails to find Jules by his promised deadline, Mary Pat goes to his house and learns from Brian's wife that Jules has been involved with a hard-edged Butler crew member named Frank Toomey.

Two detectives, Bobby Coyne and Vince Pritchard, come to Mary Pat's house and ask her questions, which she cautiously answers because she is angry with Brian. That night, she goes to the Fields, the Butler crew's hangout, and directly confronts Brian, who warns her not to draw attention to the Butler crew and chastises her for going to his house and speaking with the police. After Mary Pat leaves work the next day, she is confronted by Marty Butler himself, who hands Mary Pat a payoff and suggests she leave the state; she realizes that her daughter has been killed. Meanwhile, Coyne and Pritchard learn that Augustus was chased into the train station by four youths and mortally wounded before he was ever thrown onto the tracks. Coyne decides to make arrests, but is unable to get any information out of Brenda and Rum before Marty Butler's lawyers swoop in and ferry them away.

After several days of drinking and grieving, Mary Pat is brought to an anti-busing rally by several members of an organization called SWAB (Southie Women Against Busing), but destroys her standing in the South Boston community when she gets into a fistfight with one of the SWAB members, Joyce, after Joyce tries to hit her daughter Cecilia. Mary Pat heads home, digs up her husband's old kit bag, and resolves to resort to vigilante justice. She intercepts Coyne outside of his police station and implies that she intends to torture the people involved in Jules' death for information before passing that information on to the police; though Coyne cautions her against doing this, he does not stop her. Mary Pat spends the evening stalking Rum, then threatening him with castration if he does not speak to the police. Rum goes to the station and explains to Coyne and Pritchard that he, Brenda, George, and Jules were indeed responsible for Auggie's death. When Coyne and Pritchard press him, Rum admits that Frank Toomey was involved in the murder as well, and that he was angry with Jules because she had become pregnant with Frank's child and was threatening to tell people about it.

Mary Pat orchestrates a theft of George Dunbar's supply of drugs in order to locate him and begin tailing him, a decision that leads her to Roxbury, where she watches George exchange a bag of guns furnished by the Butler crew for more drugs with a group of Black men. Mary Pat follows George home, forces him to use his own supply of heroin before interrogating him and learning that Brian and Frank buried Jules in the concrete basement of the house behind the Fields. Mary Pat then handcuffs George to her car, drives him out to a pier, and calls Coyne to come arrest him. That night, Mary Pat sets the Fields and the adjoining house on fire before informing Coyne that Jules' body is in the basement; after Coyne locates the body, he has Mary Pat identify it, then learns from her that George was spotted giving weapons to a group of Black men in Roxbury. The next day, Mary Pat learns that Auggie's funeral is being held in Mattapan and decides to attend; at the wake, though, Auggie's parents (Calliope and Reginald) inform Mary Pat that they consider her responsible for their son's death and threaten to hurt her if she does not leave the neighborhood.

Coyne leads a raid on the Global Liberian Liberation Front, the name of the group that George was seen giving guns to, and learns that Marty Butler instructed them to initiate a race war by opening fire on groups of white students. Meanwhile, Mary Pat manages to locate Frank Toomey at an anti-busing rally and follows him across Boston before finding him outside his house, incapacitating him, and dragging him into her car. She takes him to Castle Island and interrogates him for some time before Marty and his men arrive. Coyne gets a call from Pritchard and rushes a team of police to Castle Island to try and head off the confrontation there. Mary Pat manages to kill Frank and take Brian hostage before Marty and his men corner her and Brian and kill them both. Frank and his men are arrested (though Coyne doubts they will be charged), Mary Pat's funeral is sparsely attended, and Calliope manages to make peace through a conversation with Mary Pat's ex-husband Ken.