The film begins with Daniel Lugo (Mark Wahlberg) fleeing from a swarm of heavily armed Miami police officers, only to be struck by a police cruiser. The narrative then rewinds to a few months prior. Lugo is a resourceful schemer and a recently released convict with a history of Medicare fraud. To boost its membership and focus on fitness, John Mese (Rob Corddry), owner of the Sun Gym, hires Lugo. Within just six weeks, he manages to triple the gym's membership and forms a bond with trainer Adrian Doorbal (Anthony Mackie). However, Doorbal, a dedicated bodybuilder, struggles with impotence due to steroid use.
Lugo soon becomes obsessed with the lavish earnings and lifestyle of new gym member Victor Kershaw (Tony Shalhoub), whom he perceives as an unscrupulous figure. Motivated by the rhetoric of motivational speaker Johnny Wu (Ken Jeong), Lugo adopts the mentality of a "doer" and devises a sinister plot to kidnap and extort Kershaw for his wealth through torture and coercion.
To execute this plan, Lugo enlists the help of Doorbal and Paul Doyle (Dwayne Johnson), a recent ex-convict battling a cocaine addiction who has found faith. Initially hesitant, Doyle succumbs to joining the gang after a violent encounter with his priest. The notorious "Sun Gym gang" makes an initial, unsuccessful attempt to kidnap Kershaw at home, ultimately incapacitating him with a taser outside of his deli. They take him to a small, rather unusual warehouse filled with sex toys. Although they disguise their voices, Kershaw remarkably recognizes Lugo by his unique cologne.
Despite the rocky start, their plan proceeds as intended; Kershaw complies under duress, making deceitful calls about his disappearance, moving his family away, and even signing documents that transfer his assets to Lugo. In a strategic move, Lugo bribes Mese to notarize these documents by presenting fabricated paperwork under Kershaw's name, spending his money to sponsor the Sun Gym's activities.
While the Sun Gym gang revels in their plunder, they conclude that allowing Kershaw to escape is too risky. Lugo concocts a deadly scheme: he intends to drown Kershaw in what would appear to be a drunken car accident. Yet, when Kershaw survives the crash, the situation escalates, leading the gang to burn the vehicle with him inside. Miraculously, Kershaw escapes the flames, but the gang runs him over, believing him to be dead. Unbeknownst to them, Kershaw survives and ends up hospitalized.
Upon reporting the incident, Kershaw faces disbelief from the police, who find his demeanor off-putting despite his recollections of events, especially given his elevated blood alcohol levels. He then turns to Ed Du Bois, III (Ed Harris), a retired private investigator. After initially declining the case, Du Bois advises Kershaw to leave the hospital quickly in case the gang comes back to finish what they started.
Taking this to heart, Kershaw hides in a motel. Later, Du Bois decides to take on Kershaw's case and begins shadowing the Sun Gym gang. After a suspicious encounter at the gym, Lugo feels the heat closing in on him. When Kershaw reaches out to Mese about his stolen wealth, the gang attempts to prevent him from revealing their identity by going to his motel, but they arrive too late; Kershaw has escaped.
The plot thickens when Lugo and Doorbal learn Du Bois is using his credit card to pay for Kershaw’s motel stay. They devise another kidnapping plan, this time targeting Frank Griga (Michael Rispoli), the owner of a phone sex operation. Following a promising initial meeting, Lugo's petty jealousy over being questioned leads to disastrous results when he accidentally kills Griga. In a frenzy, they inject Griga's wife, Krisztina Furton (Keili Lefkowitz), with a tranquilizer, ultimately leading to her accidental death as well.
In a desperate attempt to cover their tracks, Lugo and Doorbal set out to dismember the bodies and dispose of the remains in a lake, while Doyle incinerates their fingerprints. However, the weight of their crimes drives Doyle to madness, prompting him to abandon the gang and revisit the church.
Meanwhile, the police, aided by evidence from Du Bois, devise a plan to apprehend the Sun Gym gang. The timeline returns to June 17: police arrest Doyle, Doorbal, and Mese. Lugo, witnessing the raid, manages to flee, escaping onto the water in Kershaw's fast boat. Realizing that Lugo is heading to seize his hidden bank account in the Bahamas, Kershaw and Du Bois accompany the authorities to intercept him.
Ultimately, as Lugo attempts to escape, he is caught by Du Bois, who incapacitated him before the police take him into custody. All members of the Sun Gym gang face their respective trials; Lugo receives a death sentence plus an additional 30 days for disrespecting a guard, while Doorbal is sentenced to death. Doyle is given 15 years but serves only 7, while Mese faces a 15-year sentence and dies behind bars. To protect the survivor's identity, Victor Kershaw's name is altered, leading the film to conclude with Lugo's voice ominously stating, > "That's the American dream."
