NOTE: This spoiler was submitted by Evan B
The film opens with a cop named Marv Boswick walking through a street festival. He is pickpocketed and pursues the perpetrator through the crowd and follows him into the sewer system. Boswick is then attacked and knocked unconscious by a figure in a pig mask. When he awakens, Boswick finds himself on a subway track. He is standing on a stool, bound by razor wire, and his tongue is screwed into a vise. A TV also placed on the track turns on and he sees a video from someone mimicking the Jigsaw killer from the Saw films. The video says that Boswick used his tongue to lie in court and railroad innocent people. Now, he will be literally railroaded by an oncoming subway unless he rips out his tongue and jumps off the track. Boswick struggles and eventually rips off his tongue. But it’s to no avail as he is too slow and is crushed by the train.
We cut to a man named Zeke Banks (Chris Rock) committing a robbery with his gang. As his gang tries to escape, they are surrounded by cops. It’s revealed that Banks is an undercover agent. Because he didn’t inform his precinct of this heist, they blew his cover. Banks has no choice but to return to his role as a detective. Captain Angie Garza (Marisol Nichols) assigns him a rookie partner named William Schenk (Max Minghella).
Banks has a fraught relationship with the other cops in the precinct. Schenk learns that they despise him because he turned in a dirty cop. Banks also feels inferior because his father Marcus (Samuel L. Jackson) was the precinct’s old campus and a beloved cop. Schenk remarks that Marcus is the reason for everything he does.
Banks and Schenk investigate the killing in the subway, not knowing that the corpse is Boswick. The pair also bond, with Banks telling Schenk that he is divorced and doesn’t trust women. Schenk replies that he is happily married with a young son.
After they return to the precinct, Banks gets a package with a USB drive. On it is a message played by the new Jigsaw killer. The killer uses a pig marionette dressed in a police uniform as his avatar. The killer explains that he intends to reform the police through his actions. It ends with an image of a spiral painted on the wall of the courthouse. The cops go to the courthouse where they find another box. Inside is Boswick’s tongue and badge. The police now realize the killer is targeting them and that Boswick was killed in the subway. Banks pays his respects to Boswick’s widow and has an argument with his ex-wife who is also there.
Banks goes home for the night where his father is waiting for him. The two have a distant and strained relationship. Nonetheless, Marcus promises to help Banks with the case over dinner the following night.
In a flashback, we learn that the officer Banks turned in for being dirty was named Pete. Banks witnessed Pete shoot and kill a potential witness in a murder case against a cop. Due to Banks’ testimony, Pete went to jail for nine years.
The next morning, Detectives Kraus and Fitch (Richard Zeppieri) use video footage to identify the man Boswick was chasing before he was murdered. They decide not to tell Banks as they hate him. Fitch goes to a warehouse where he believes the suspect lives, but finds nobody there. Instead, he is knocked unconscious by a cloaked figure. When he awakens, he finds himself in a trap.
That night, Banks goes to his father’s home to work on the case but finds him gone. A flashback shows that Marcus disapproved of Banks turning in Pete, but still did his best to protect his son from the inevitable blowback.
The next day, another package from the Jigsaw killer is delivered to Banks revealing a new location. The police venture there and find Fitch’s badge and his ten severed fingers. A flashback shows us what happened. Fitch once gunned down an innocent man after the man insulted him during a traffic stop. The Jigsaw killer has placed Fitch in a tub slowly filling with water. At some point, the water will hit exposed electricity wires and electrocute him. Each of Fitch’s fingers is attached by a wire to a winch that is activated whenever Fitch clenches his teeth on a mechanism. The Jigsaw killer explains in a recording that Fitch used his fingers to murder people, so he must now rip them off to live. The detective tries to rip off his fingers but, like Boswick, doesn’t do it fast enough to escape death.
In another flashback, we also learn that Fitch refused to provide backup for Banks out of anger for what happened with Pete. As a result, Banks is shot while pursuing a suspect. Marcus punches Fitch for his insubordination, but doesn’t remove him from the force.
Banks and Schenk continue the investigation but find no leads. Banks decides to interrogate Pete, who is now leading a recovery group for ex-convicts. Pete is regretful for killing the witness, but still bitter at Banks for turning him in. Nonetheless, he has an alibi for the murder. Meanwhile, Marcus has still never returned home.
The next day, Banks gets another package. Inside this one is Schenk’s skin (Banks recognizes it due to the skin containing a tattoo from Schenk’s arm). In a recording, the killer says he is under Banks’ skin and he will soon go for the head. There is also another clue linked to Banks’ past inside. Banks is devastated that his new partner is dead
As Banks goes to tell Schenk’s family of his death, the police scanner announces an attack by the Jigsaw killer on a beat cop. Banks rushes to the scene where the cop says that he was slashed by a person in a pig mask. Banks is confused as this doesn’t match the Jigsaw killer’s MO. He eventually deduces that this attack was simply meant to distract the police. He also realizes that when the killer said it was going for the “head,” he meant Chief Garza as the head of police. Banks can’t get in touch with the chief and rushes back to the station.
Simultaneously, Garza gets a text telling her to get a file from the cold case storage room that will help her crack the Jigsaw case. The message lures her into an ambush and she is apprehended by the killer and attached to a trap. The killer’s recording explains that Garza turns a blind eye to the rampant corruption in her precinct and she must be punished. A faucet above her head begins dripping scalding hot wax onto her face. She can deactivate the trap if she jabs her neck against a blade and sever her spinal cord. Otherwise, the wax will silence her forever. Garza is killed by the wax, with Banks arriving a few moments too slow to save her.
Banks blames Pete for the murders, while another cop blames Marcus. Banks is unsure what to do as he has no idea where his father is. But he pursues his initial lead and goes after Pete. When he arrives at Pete’s workplace, he is knocked unconscious by the killer.
We cut to the recent past and see that Marcus found the killer’s headquarters. Marcus is attacked by the killer and after a skirmish, he is knocked out.
When Banks recovers, he is handcuffed to a pipe with a hacksaw nearby. Banks readies himself to saw through his hand when he sees a bobbie pin on the floor nearby. He uses the pin to pick the lock. Banks learns he is locked in a room with a captured Pete. A recording plays telling the duo that Pete’s life is in Banks’ hands. A crusher machine is then activated and glass bottles are sent into the machine via conveyor belt. When the bottles are crushed, the shards are shot out of the crusher and into Pete’s back. Banks finds the key to release Pete and is wounded by the glass in his efforts to free him. Although Banks frees Pete from the trap, Pete dies from his wounds.
The door opens and Banks walks out to find… Schenk. It turns out that Schenk is the new Jigsaw killer. We learn that Schenk was the son of the witness that Pete murdered. Banks made sure that Schenk wasn’t also killed after helping him stay hidden after his dad was shot. Schenk explains that he faked his family so that people wouldn’t suspect him as a killer and that the skin with the tattoo on it was actually Boswick’s skin that he tattooed after Boswick died. Schenk also explains that he was inspired in this mission by John Kramer, the original Jigsaw killer.
Schenk tells Banks that he has his father (he pickpocketed Banks’ phone and texted him the address to the ambush). Schenk then asks Banks to join him in his mission of killing dirty cops. Banks says he will join Schenk, but the killer doesn’t trust him. Schenk says that Banks failed the first test by trying to save Pete. He will now give Banks a second test to see if Banks can be trusted.
Schenk calls the police and reports an active shooting at their location. He then leads Banks into a room where Marcus is strung up by wires from the rafters and hands Banks a gun with one bullet. Schenk has been slowly draining Marcus’ blood. The test is that Banks can shoot a mechanism that will allow his father to be freed. If he does that, Schenk will make an escape through a hidden elevator knowing that Banks is not an ally. Banks can also shoot Schenk and stop him, but will then have no bullets remaining to free Marcus and Marcus will die of blood loss. Or Banks can escape with Schenk, thus proving his commitment and become Schenk’s partner. Schenk explains that Marcus is being punished because he was the chief who presided over a reign of terror, letting his precinct engage in all manner of illegal behavior in the name of being tough on crime. When Schenk said everything was due to Marcus, he meant that Marcus is indirectly responsible for his father’s death. Marcus objects to this portrayal and claims his tactics did more good than harm. Meanwhile, the cops arrive and begin storming the warehouse.
Banks eventually uses the bullet to activate the mechanism to free his father. Marcus, though still attached to the wires, is lowered to the ground. Banks pulls out the tubes draining Marcus’ blood as Schenk runs to the escape elevator. Banks manages to catch up to Schenk and the pair engage in a fistfight. The cops bust through the door and rush in. The wires suddenly activate again, manipulating Marcus like a marionette (the same type of puppet Schenk used to deliver his messages as the Jigsaw killer). The wires make it look like Marcus is aiming a weapon at the cops. Banks abandons Schenk to try and call off the cops, but they open fire. Marcus is killed in front of his son. As Banks screams out in rage, Schenk disappears in his elevator with a smile on his face.