A boy locked in a tool shed fights off a dog biting his ankle until he manages to kill it with a hoe. The body of a young woman dressed as a bride is found in an abandoned swimming pool, and Zárate, a young inspector, notices that maggots are crawling out of incisions on the victim's head. Two BAC agents — the Criminal Analysis Brigade — take over the investigation, and the inspector doesn't take kindly to being sidelined.

The boy remains locked up and crying, unable to escape. The wound on his ankle is getting worse and the dog lies dead beside him. The camera at Inspector Blanco's house continues photographing the town square. The BAC questions Cintia, who reported her friend's disappearance, but she points them towards her fiancé, with whom she was using drugs. Chesca and Orduño discover that someone paid the homeless people who frequented the area to disappear, but despite Chesca's pressure on El Tuerto — the ringleader among the vagrants — they can't get any further. Moisés, the father, wants to bury his daughter and find out who her boyfriend was, but the investigation delays this. His wife Sonia notices that a small chain with a cross her daughter always wore is missing from her belongings. Zárate updates the commissioner on their visit to Salvador, who reminds him why he's on the case.

The locked-up boy rummages for something to eat and finds a tin of peaches in syrup, but cuts himself trying to open it. Blood, syrup and tears mingle on his face. While the Macaya family holds the funeral for their daughter, Chesca and Orduño tail Raúl and, despite his attempt to flee, manage to detain him. Searching his home, the forensic examiner recognises the bed from which Susana used to take photos of herself imitating her dead sister. Sonia, the deceased's mother, reproaches her husband for allowing El Capi to attend the funeral — she doesn't want him near. Raúl is uncooperative during questioning, though he eventually admits he followed his fiancée that night to make sure she wasn't cheating on him. Elena and Zárate decide to pay Cintia another visit, only to find she has slashed her wrists and is bleeding out in the bathtub. They manage to revive her. They discover photos of Susana and Cintia engaged in sexual acts, with the word whores written across them.

The locked-up boy discovers maggots in the dead dog — and also in the wound on his ankle. Fascinated, he begins to eat them. Chesca and Orduño look for clues about the attack on Miguel in prison but only find that the nine stab wounds came from the Romani wing. Elena, focused on keeping watch over Lusson, ignores her team's calls. Commissioner Rentero warns Zárate about the dangers of reopening the Lara case, and the furious inspector makes the mistake of visiting the prisoner in hospital without the proper authorisation. The new lawyer Masegosa prevents Elena from questioning him — thanks to her meddlesome colleague's blunder. Elena tracks Lusson, determined to find her son no matter what. Sonia reproaches her husband for not telling her about the private detective, and a sullen Moisés leaves to carry out a job for his cousin El Capi.

DNA evidence confirms the BAC's suspicions. Moisés is now a wanted man, wrestling with his inner demons. Elena speaks with Jáuregui, Vistas's defence lawyer, who has doubts about the evidence used to incriminate his client. Analysing the evidence from the Lara Macaya case years earlier, Buendía spots inconsistencies suggesting the whole thing may have been a setup orchestrated by Salvador Santos. Zárate's behaviour raises questions about where his loyalties lie, and he is removed from the case.

The interrogation of Santos brings to light the mistakes the police made in the Lara Macaya case. Elena then confronts her superior, Rentero, who strips her of the investigation. A search of Vistas's cell uncovers letters sent by a former fellow inmate, Candela, referencing a ritual connected to the murders of both Lara and Susana. Chesca and Orduño, joined by Elena, head to the unsettling squat where he lives, while Zárate pays a visit to Jáuregui — Vistas's lawyer — and questions him about the inconsistencies in his defence strategy at trial.

The interrogation of Jáuregui clears up many of the unknowns surrounding the Lara and Susana cases, but forensic examiner Buendía identifies differences between the two murders that sow doubt within the BAC. Elena finally seems ready to move on from the disappearance of her son, channelling all her energy into closing the case — and uncovering revealing hidden connections between Jáuregui and Vistas, who is ultimately released from custody.

Jáuregui finally reveals the whole truth about Susana's murder. Vistas's shattered childhood in an orphanage sheds light on his obsessions and allows the team to anticipate his next move. The team launches a relentless manhunt to catch the killer before more crimes are committed. Chesca and Orduño then discover a wardrobe containing numerous hard drives filled with videos of people being tortured — including one that shakes Elena to her core… and signals the beginning of a new case.