Luther discovers Ripley's corpse and is charged with his murder by Stark and Gray, but whilst in transit the police car is attacked and Luther is rescued by Alice Morgan. After Marwood goes on the rampage at Luther's flat Superintendent Schenk is aware that he killed Ripley and Mary is taken to a hideaway by Stark and Gray, though Gray starts to sense that Stark wants to reel Luther in for a confrontation. Marwood meanwhile abducts the wife of the doctor at the prison where Caitlin's murderer is held and threatens to kill her unless the doctor gives him a lethal injection. Alice and Luther track Marwood down but he escapes, locating Mary via Luther's phone. He kills Stark and wounds Gray but Alice and Luther arrive for a roof-top showdown.
After Tom Marwood kills DS Justin Ripley and attacks Mary Day, DSU George Stark and DCI Erin Gray arrest Luther on suspicion of murder. Their hypothesis is that when Luther met Marwood a few days before and let him go, Luther struck a deal to have Ripley and Mary killed as they could both provide evidence against him. While they are taking Luther by car to another location they are attacked - and Luther is freed by none other than Alice Morgan who acts as an intermediary with DSU Schenk. Marwood's target is the imprisoned man who killed his wife. To get to him he kidnaps the prison doctor's very pregnant wife. Luther is running out of time as he tries to find Mary, evade capture by DSU Stark and capture Marwood alive.
- Luther sits by Ripley's corpse, stunned.
Marwood shoots open the door to Luther's house and enters. Mary hides and Marwood, unaware that she's there, begins shooting things. Marwood hears Mary as she flees the building. She hides in an outbuilding and Marwood almost finds her but he flees when he hears police sirens.
Stark meets with Mary in an ambulance. He asks her what Luther said and she tells him about how he asked to meet her there tonight. She realizes that Stark is implying that Luther sent the man there to kill her. She denies this. Gray comes in and tells them that Ripley has been killed. Stark wonders why the people who Luther loves always seem to meet violent ends.
Luther and Schenk arrive at Luther's house to find it destroyed. Gray and Stark show up and arrest Luther for the murder of Ripley and the attempted murder of Mary.
At the precinct, Stark and Gray interrogate Luther. Stark explains that two people who could incriminate him were attacked by Marwood, and that Mary only survived because he intervened. Gray notes that after his confrontation with Marwood, Luther "let him go" and then wonders what the quid pro quo was. She and Stark suggest that in exchange for giving Marwood more time to go on his crusade, Luther enlisted him to kill people who could identify him.
As Gray and Stark transport Luther in their car, their tires are blown out by a spike strip. When Stark gets out to investigate, a smoke bomb is trown at him. A hooded figure wearing a gas mask approaches and hits Stark with a baton and then sprays him with pepper spray. The figure then sprays Gray with pepper spray, incapacitating her. They get in the back with Luther. The figure pulls off their gas mask to reveal that they are Alice Morgan.
Marwood listens to a news report about Ripley's death, angered and saddened about his actions.
Alice takes Luther back to her apartment. She claims to have been in Berlin and gives Luther a passport. He tells her that "after Marwood I'm done." Alice notes that time again Luther's actions have killed people he professes to love and says "your conscience has killed more people than I have."
Schenk and Stark meet at the scene of the attack on the police car. Schenk disagrees with Stark's description of it as an 'escape' saying that it seems more like a violent abduction. Stark says that if Marwood had done it looking for revenge he would have just killed Luther. Schenk says that they're still searching for Marwood.
A pregnant woman watches a news report about Milan Nizac, the man who killed Caitlin Marwood, being attacked in prison. Tom Marwood comes to her back door and breaks in.
Gray takes Mary to a safehouse. Alice tells Luther that she got married while she was away to a man named Bertrand she met in Sao Paulo. Luther notices her use of the past tense and she says that he died. She comments "some little girls want ponies. I always wanted to be a widow." She asks if Luther has found anyone and when he tells her yes she says that the woman is a "pixie" just a dream of a life Luther wants to live. She tells him that the "pixie" isn't what he wants but what he wants to want. When Luther says that her name is Mary, Alice says "I know." Alice turns on a news report about Ripley's killing. She describes Marwood as a "needy little fishy". Luther says that he plans to "take away" everything Marwood wants: the chance to avenge his wife, the admiration of the public. Alice approvingly describes Luther's plan as "cruel". Luther asks Alice for help figuring out how Marwood plans to kill the imprisoned Milan Nizac.
A man returns home to find his house ransacked and his wife, Nat, missing. He receives a phone call from Tom Marwood, who tells the man that if he does what Marwood asks, his wife will be returned to him.
Alice goes to the police station and meets with Schenk. He tells her that since she's a fugitive he should lock her up. She says that if he does that, Luther will die at the hands of Tom Marwood. She says that Luther is as innocent as she is culpable. She passes on a message from Luther that Tom Marwood will not allow himself to be arrested. Instead he plans to commit "suicide by cop" in order to become a martyr. Luther has a plan to bring Marwood in alive, but he will need Schenk's help. She asks for information on Milan Nizac and a personnel file.
Luther tells Alice he knows she's been in London for weeks and asks why she's here. She says that she wanted something: him. Alice derides Luther's attraction to Mary.
Luther calls Mary. Stark has her answer the phone so that he can track it. Luther tells her that he hasn't done anything that they've accused him of. He says that Stark means well and Grey is a good woman who Mary can trust. Stark gets a lock on Luther's location. Luther in turn gets word from his office about where Stark and Gray are. Alice has gone through the police files and discovered that Nizac is in the prison hospital where he's attended by a Dr Joe Green.
Luther goes to Green's house where Green attacks him saying that Marwood said no police. Luther subdues him and Green explains that Marwood told him to kill Nizac with a lethal injection or Marwood would do to Natalie what was done to Caitlin. Luther promises to get Natalie back and has a trace done on the last number to call Joe Green.
Marwood has a man named Ryan Clark tied up. He records a video explaining that Clark was high on meth and driving a stolen car when he ran over and killed a twelve year old boy. Clark got only five years in prison. Marwood explains that he has made a list of offenders and he will kill one every day until the Prime Minister agrees to hold a referendum on the death penalty. Marwood says he wants to give the people a choice. Luther appears and says that Marwood didn't give Justin Ripley a choice. Marwood tries to blame Ripley's death on Luther but Luther says that he didn't ask Marwood to kill Ripley. Marwood sees Alice holding the video camera and demands that she give it to him or he will shoot Luther. Alice says he won't because he needs to be admired and people hate a cop killer. Luther asks Marwood to give up and tell them where Natalie Green is. He explains that Natalie never hurt anyone and is pregnant. Marwood pulls out another gun and has Alice put the camera on the ground. Alice flees and Marwood takes the camera. Luther explains that the whole world will now see him for a cop killer who kidnapped a pregnant woman, and Marwood realizes that Alice took the tape with her. Luther promises to give the tape back if Marwood will release Natalie. Marwood calls him a liar and shoots him in the leg. Marwood then reminds Luther that he stole his phone and so he knows all about Mary Day. He demands that Luther give him back the tape and he will give Luther back Mary.
Luther calls Mary and tries to warn Stark that Marwood is coming, but Stark hangs up, thinkign that Luther is just trying to lure them out of their safehouse. Marwood arrives at the building where they're keeping Mary. Alice kicks an empty water jug down the stairs creating a noise and alerting Stark and Gray. The two detectives draw guns and proceed into the hallway, thinking that Luther has come to attack Mary. Meanwhile, Alice sneaks into the safehouse and accesses Stark's computer, downloading the video file onto it. Alice threatens Mary that if she ever betrays Luther again Alice will kill her.
In the stairwell, Tom Marwood shoots Stark in the chest. He holds Gray at gunpoint and asks if she's Mary. Gray replies yes, and asks if Luther sent him. Marwood is confused, but then realizes that Gray's voice doesn't match the voice of Mary which he heard on the phone. He then shoots her. Alice realizes that Marwood is coming and locates a magazine of bullets but no gun. She and Mary escape out a back stairwell. As Marwood approaches, Alice wedges a bullet between two doors and uses a nail to fire it, injuring Marwood's arm. Marwood catches up to them, however, Luther has arrived and Marwood forces Mary and Alice to flee with him to the roof.
Marwood holds Alice and Mary at gunpoint on the rooftop as the police arrive. Luther tries to talk him into revealing where Natalie is. Marwood demands thatg Luther choose whether Alice or Mary lives, but Luther refuses. Marwood presses, saying that he will kill both if Luther doesn't choose. At a slight nod from Alice, Luther agrees, and says that Marwood should kill Alice. When Marwood raises his gun to Alice's head, Mary knocks it out of the way and Alice turns around and stabs him in the neck with the nail.
Luther tells Mary and Alice to run and puts pressure on Marwood's wound. He tells Marwood that if he lets go Marwood will die, which he knows is what Marwood wants. He tells Marwood to beg him to let him die and demands to know where Natalie is. Marwood tells him that Natalie is on Cathay street but Luther refuses to let him die. Schenk and the cops free Natalie. Luther sees Erin Gray as she's being loaded into an ambulance and is informed that Alice has been arrested and is given a note from Alice. When he goes into the paddy wagon to say goodbye to Alice he finds Mary who explains that she pretended to be Alice in order to let the other woman escape. Luther says a tender goodbye to her and walks off. He meets Alice on the bridge where they first confronted each other years before. She says that his choice on the rooftop was clever and asks "now what?" The two of them walk off together.