NOTE: This spoiler was submitted by Jeremy

The film opens with Detective Spenser (Mark Wahlberg) arriving with his partner Driscoll (Bokeem Woodbine) to the home of Captain John Boylan (Michael Gaston). When Spenser barges into Boylan’s home, he sees the man is drunk, and his wife looks bloodied and battered. As heard in a testimonial voiceover to a judge, Spenser explains that he went to Boylan’s house to question him on covering up the murder of a woman named Gloria Wiesnewski (Alexandra Vino). The confrontation got heated, and Spenser beat Boylan senseless, insisting that he had it coming.

Five years later, Spenser is almost out of prison. He is approached by an inmate named Squeeb (Post Malone), who is part of an Aryan supremacist prison group. He and the other Aryans gather around to give Spenser a “going away message”. Spenser then ends up in a brawl where he fights all of them off before he is finally released.

Spenser is picked up by his friend and associate Henry Cimoli (Alan Arkin). To his horror, Spenser’s ex-girlfriend Cissy Davis (Iliza Schlesinger) shows up to pick him up at the prison, but Spenser and Henry run into Henry’s car to drive away. Henry brings Spenser to his new home where his dog Pearl still lives, and he now shares a room with a man named Hawk (Winston Duke). To Spenser’s dismay, Pearl seems to have taken more of a liking to Hawk than to be ecstatic to see him.

Later that night, Boylan is going home while talking to his daughter on the phone. He drives near a bus yard where he gets hit by a truck. A group of hooded men come out and pull Boylan out of his car, beating him mercilessly, which his daughter can hear. They accuse Boylan of talking to someone about something they did, and one man kills Boylan with a machete. The following morning, a nurse named Letitia Graham (Hope Olaide Wilson) comes home to find her husband Terrence (Brandon Scales) dead in the front seat of his car with a gunshot wound to his head. Letitia breaks down hysterically as onlookers come by to help.

News spreads of Boylan’s murder, with Spenser immediately becoming a prime suspect. Driscoll and another detective visit Spenser to confirm that he wasn’t near the crime scene when Boylan was murdered, but every other cop in the city hates Spenser’s guts and thinks he had something to do with it. Spenser then goes about his day by starting a new job as a truck driver, but after seeing that Terrence is being accused of involvement in the murder, as well as the media’s belief that he committed suicide out of guilt and Letitia in distress on TV, Spenser takes it upon himself to solve the case, since he personally knew Terrence and knew he was a great cop. Spenser visits Letitia, who says the detectives that raided her home appeared to know exactly where to find a stash of drugs that had clearly been planted there. She informs Spenser that Terrence was last known to be going to a cop bar with Boylan.

Spenser goes to the cop bar and speaks to another detective who would have known about any involvement Terrence had with Boylan. Spenser sincerely apologizes for how he handled things with Boylan, but a few other cops loyal to Boylan enter and begin to fight Spenser before throwing him out. He then goes to a nearby bodega where there is security cam footage of Terrence and Boylan meeting outside the bar. He brings it home to inspect with the help of Hawk.

Spenser later visits the crime scene to look for anything that may have been missed. He visits Driscoll at a gym, where he denies having been at the crime scene, but Spenser suspects otherwise since he found a toothpick there, which Driscoll is known to usually have in his mouth.

Spenser and Hawk have lunch together, but Spenser is displeased to see Cissy is there too. She follows him into the bathroom where they end up having quick sex. She brings up how they had a pregnancy scare, and Spenser insisted he would be there for the child even if they didn’t get married, but Cissy is glad that she didn’t get pregnant. After they leave, Spenser and Hawk spot the same Corvette that was seen in the security footage. Spenser chases after it on foot and cuts through a yard where he is attacked by a guard dog. However, Hawk managed to get the license plate number. Spenser takes it to the DMV and forces the man behind the desk to give him the information. The car belongs to an Irish mob enforcer called “Tracksuit” Charlie Bentwood (James DuMont).

Spenser, Hawk, and Henry sit outside to keep an eye on Bentwood as he is getting a massage. Here is where Spenser explains his whole history with Boylan and Gloria Wiesnewski: he used to shovel snow off her mother’s porch when he was younger, and Gloria was an activist against gentrification. Bentwood was sent to scare her off, but she wouldn’t budge. She would later go home to find her cat nailed to her door before Spenser later came across her body lying in a trunk. He had to break the news to Gloria’s mother himself. He saw security cam footage of thugs attacking and murdering Gloria, but later found out that Boylan was just sitting on the murder case and sent Spenser out when he confronted him over it. This is what led to Spenser attacking Boylan in his home. Hawk then vandalizes Bentwood’s car to get a rise out of him. They continue to scope out Bentwood for a few days until Spenser is told by two FBI agents to back off the case.

Spenser visits Squeeb in the hospital since he knows he might have some answers to further solving the case. Squeeb refuses to talk until Spenser shows him his phone where Henry is recording Hawk on a date with Squeeb’s girlfriend. Squeeb gives in and gives Spenser only one word: “Wonderland.” With help from reporter Wayne Cosgrove (Marc Maron), Spenser and Hawk learn that Wonderland is a casino syndicate run between dirty cops and the Trinitarios gang. Spenser goes by the area where he is confronted by two Trinitarios, but they make him go away.

Boylan is given a traditional officers’ funeral, while Terrence is only given a small one with close family and friends. Driscoll talks to his partner, Macklin (Kip Weeks), over a situation unfolding between them that could lead to some unwanted exposure.

Spenser goes to a restaurant where the Trinitarios spot him. They attack him with machetes but Spenser manages to fight them off before Hawk crashes in with the car. Knowing that the Trinitarios know who he is and can target him, Spenser brings Pearl and Hawk to hide out with Cissy. He later goes to Letitia’s home after it’s been vandalized, and she gives him an audio recording that Terrence hid between himself and Boylan. The tape confirms that Boylan was a dirty cop, and that Bentwood, Macklin, and Driscoll all worked together, and it was Driscoll who struck the killing blow to Boylan. Spenser brings the tape to the FBI, but they deem it insufficient evidence to go after Driscoll.

Spenser finds Bentwood and violently interrogates him. He confesses that there’s a shipment of drugs coming from New Hampshire to Wonderland. Spenser and Hawk find the truck and run it off the road, killing two of the crooks inside before coming across the stash. Spenser is later contacted by Driscoll, who knows that Spenser knows about him, and so he has kidnapped Henry and forces Spenser to meet at Wonderland within an hour.

Cissy joins the two on their mission, and she helps Henry get free. Spenser and Hawk ride in on a truck to smash the criminals’ cars before proceeding to fight them all off before it’s just Spenser and Driscoll. The two have a brawl where Driscoll almost wins, but Spenser fights him and stops short of beating him the same way he beat Boylan to avoid returning to jail. They leave the dirty cops outside with all the evidence, just as the police arrive to arrest Driscoll and his goons.

Terrence is exonerated and is set to be given a proper officers’ funeral, as well as a scholarship being set up for his son. Spenser, Hawk, Cissy, and Henry go out to celebrate for lobster, until Spenser sees a news report of a church fire from two years earlier, and the Boston fire chief is under arrest as a suspect, despite insisting he is innocent. Spenser claims he knew the guy, which gives Henry and Cissy the bad feeling that Spenser is going to take on this case as well.

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Detective Spenser has served five years in prison for assaulting corrupt Captain John Boylan. Upon his release, Boylan is found murdered, and despite Spenser being the prime suspect, attention turns to another deceased cop named Terrence Graham, who was allegedly involved in the murder and committed suicide afterward. Spenser teams up with his roommate Hawk to take on the investigation.

It turns out that Boylan and another group of dirty cops were working with the Trinitarios gang in a casino crime syndicate. Boylan told Terrence about it, and Spenser's former partner Driscoll led the dirty cops to murder Boylan and Terrence. Although they have audio proof of this exchange between Boylan and Terrence, the FBI dismisses it as insufficient evidence.

After finding a stash of drugs headed for the casino, Driscoll kidnaps Spenser's associate Henry and forces him to meet at the casino. Spenser and Hawk, as well as Spenser's ex-girlfriend Cissy, all arrive to free Henry and fight the villains, ending with Spenser fighting Driscoll and leaving him and the evidence to be found, leading to his and the other dirty cops' arrests.

Terrence has his name cleared posthumously, and his family will receive benefits. As Spenser and his friends celebrate, he sees a news report about a fire chief he knew being accused of burning down a church, leading Spenser to want to take on this case too.