Plot

A bouncer with a slightly murderous anger-management problem that she controls with the help of an electrode-lined vest she uses to shock herself back to normalcy whenever she gets homicidal.

A bouncer controls her slightly murderous anger-management problem with the help of an electrode-lined vest she uses to shock herself back to normalcy whenever she gets homicidal. After the first guy she's ever fallen for is murdered, she goes on a revenge-fueled rampage to find the killer while the cops pursue her as their chief suspect.

Lindy Lewis (Kate Beckinsale) is a young woman afflicted with intermittent explosive disorder. This causes her to react with violent rage at even the slightest transgression because her body has an unusually high level of Cortisol, which in turn makes her stronger and faster than normal. Her parents eventually have her institutionalized due to her violent tendencies. After they fail to control her, the organization puts Lindy in the army, but they are also unsuccessful at controlling her rage. Finally, Lindy is subjected to an experimental (but effective) treatment to control her rage; a form of self-administered shock treatment given through an electrode-lined vest she must wear at all times, allowing her to live a normal life.

Some time later, Lindy is prompted to go on a date by her therapist (and the vest designer), Dr. Ivan Munchin (Stanley Tucci). Munchin believes that, believing this will test if she is able to have a healthy, normal relationship with someone. Eventually, Lindy decides to go on a date with an accountant named Justin (Jai Courtney). While on the date, Lindy beats their waitress (who was giving a hard time to Justin, just because he had allergies to nuts (and almost everything on the menu had nuts)) and leaves, returning to Dr. Munchin to have her electroshock remote fixed. Munchin reveals that there is nothing wrong with the remote; she has merely built up a tolerance to the electricity and ups the dosage. Dr Munchin tells Lindy that she likes the guy and encourages her to go on a date again, and have a real relationship.

Lindy contacts Justin to go on a second date. They discuss their past (Lindy was a bouncer) as well as Justin's job. Justin reveals that he only has one client; a man named Barry. They go back to Lindy's apartment and start to have sex. Justin sees Lindy's voltage therapy vest and electrodes all over her body, but he tells her it is not a big deal, and begins taking her vest off.

The next morning, Justin gets Lindy a gift along with breakfast in bed. Lindy opens the gift to see a camera, bought by Justin due to her mentioning that she likes to take pictures of people. Justin asks her to explain her voltage therapy. After explaining her condition, Justin appears to accept it. Justin then leaves for work and invites Lindy over for dinner later. Lindy visits Dr. Munchin to tell him that his theory worked and that she likes Justin a lot. Munchin insists that they have a session to deal with these feelings, but Lindy leaves to take pictures with the camera Justin gave her. Later on, Lindy calls Justin's phone after not hearing from him all day and a Detective Vicars (Bobby Cannavale) answers. She goes to the police station to sit with Detective Vicars and Detective Nevins (Laverne Cox) and they state that Justin was murdered.

Lindy replays her time with Justin and begins to spiral, shocking herself repeatedly with the vest. She visits Munchin, claiming that the vest is not working, so he gives her a sedative. Once she comes to she says she needs to find out who murdered Justin. He tells her to let it go before it changes her irreversibly. Lindy then calls the detectives claiming to have more details about Justin's murder, as a ploy to get them out of the precinct so as to gain access to the evidence room from Justin's murder. Lindy then heads to Justin's house and finds his phone. She is called by Detective Vicars who tells her that they found her and that Detective Nevins is on the way to arrest her. After a brief encounter with Nevins (Nevins tries to shoot Lindy, but Lindy distracts her and escapes), Lindy takes Justin's car and after a brief car chase in which Vicars is injured, she escapes. Lindy goes to an electronics store to talk to someone about hacking Justin's phone and meets Andi, a female hacker (a little girl) working in the store. Andi is able to locate Justin's client, Barry Kasparzki, a notorious gangster who was confirmed to be the last person in contact with him.

Upon finding out that Barry is in an abandoned building overseeing an underground fight, Lindy goes to that fight and demands that Barry tell her who killed Justin. He agrees, on the condition that she fight his top three guys all at once. Upon winning however, Barry tries to get her killed. But, Lindy manages to incapacitate both Barry and the gunman and gets him to tells her who killed Justin after threatening him with jumper cables. Barry tells her that a billionaire recluse named Gareth Fizel (David Bradley) (And Barry's boss) had Justin killed; she electrocutes him with the jumper cables anyway. Lindy visits Detective Vicars, now in the hospital after their car chase earlier, and asks him about Fizel (Vicars tells her Fizel's location as he is convinced that Lindy cared for Justin). However, Detective Nevins interrupts them and chases Lindy to the maternity ward, where Lindy begins throwing infants at Nevins to slow her down.

Lindy makes her way to Gareth's building where his head of security, Delacroix (Ori Pfeffer), knocks her unconscious. She awakens in a white room, zip-tied to a chair. Delacroix has also removed her vest. After threatening to kill her, Delacroix puts Lindy in a car back to her apartment and keeps the electroshock vest. Back at Fizel's building, Fizel tells Delacroix to get rid of Lindy (after he hears that Lindy was asking for Justin and suspects that Fizel killed him). Back in the car, Lindy tells the driver to let her out of the car, but he does not comply, so she grabs him and begins to choke him causing him to crash. Lindy then goes to Dr. Munchin to get a new vest and returns to her apartment. She notices the 9 sign on her door has been moved, showing that someone was there, so she sneaks in her window and notices that Fizel's men have rigged an explosive device to the doorway. She then disarms the explosive and pockets it for future use. She leaves, sees Detective Vicars outside her apartment, and gets in his car. She tells him he cannot arrest her because of her condition and he would be safer not to. He explains he will go easy on her because she is different and that it has nothing to do with what Nevins says about her being a woman. Her apartment blows up and she leaves.

She finds Fizel (she is captured by Delacroix when she tries to climb into Fizel's building); he tells her he is sorry that he killed Justin. He says he is glad he was able to see her before Delacroix hits her in the head again and she is back in the drainage room. He asks her if she knows what Exsanguinate means (Drainage of one's blood) and then explains it while doing it to her. She breaks free and fights him. She turns to ask how to get to Fizel before hurting Delacroix. She then fights off a few guards. After using Delacroix's finger to access Fizel's lair, Lindy learns that Justin isn't dead but in fact Justin used her as a decoy so he himself could kill Fizel (Justin works for the CIA and could not touch Fizel officially and hence used Lindy to get close to him). Justin tells her Dr. Mucnhin had something to do with everything (CIA funds his experiments) and that she was only an experiment. She goes after him and he detonates her voltage electrodes (he has the remote and he has increased the voltages). The vest becomes ineffective because she is highly upset. Lindy throws him a bag that has the bomb in it, blowing up Fizel's entire lair as she rides the elevator down.

Lindy waits for Dr. Munchin in his office ready to kill him for betraying her. Just as she decides against it and handcuffs him, Detectives Vicars and Nevins enter the office to arrest Dr. Munchin on various charges. Lindy returns to her apartment and a woman (Susan Sarandon), one of the doctors from Lindy's past, tells her that by embracing her rage she will truly become powerful. The woman then presses the voltage detonator.