Criminologist Daniel Parodi is woken when the nursery rhyme whistled by his daughter's killer during her murder plays on his computer. Realising the killer isn't done, he assembles a team to hunt him.
Malena Sanz reveals she has met Amalia, a woman with a sect tattoo claiming to have escaped white slavery. Attorney Diana Quaranta asks Parodi's team to verify the suicide of alleged slave boss Funes.
Malena and Montes track down Amalia, persuading her to testify, but then she runs away. Montes and hacker Diego find the key to the riddle in a literary text. Can they predict the Monster's next move?
When a patient of Marcos Setton, Parodi's psychologist, jumps from a balcony, Setton receives a cryptic communication from a messenger paid with forged money. The team get a shock at Funes's grave.
Parodi finds Quaranta raped at the swimming pool with another quotation written in her blood. He follows a lead to meet with head counterfeiter The Artist, but what he encounters is far more horrific.
The team identifies a licence plate reflected in a window in one of the crime scene photos. This leads to the brothel described by Amalia, but all they find is another message and a home-made bomb.
Using new clues from a literary text, the team predict that the Monster is going to commit suicide in a German church. But when Parodi confronts his tormentor, the situation is taken out of his hands.
During the autopsy, a note on Amalia's body explains the how but not the why of the crimes. At the swimming pool, a mysterious woman tries to seduce Parodi, but a very revealing call interrupts them.
The team track the man who was used to fake Funes's suicide. When the Monster's killer is revealed, it seems Parodi's past actions are the key. Quaranta is prepared to act as bait to catch her rapist.
Lydia invites Parodi out. Setton, Quaranta and Parodi hatch a plan to catch the rapist. A romantic spark between Setton and Malena is revealed. Amalia's parents leave her laptop for Diego to examine.
A case of mistaken identity puts the hunt for the rapist in jeopardy. Diego discovers the car in the window belongs to Funes's brother-in-law. Malena goes to bed but an unwelcome visitor is waiting.
Diego tells Malena how he feels but events take a hideous turn, with consequences for Montes. A cover-up begins with Lydia's help. Parodi suspects that Malena is part of the sect when she disappears.
Parodi and Diego seek Montes's notebook but find nothing. Setton is convinced Diego's hiding something, but Parodi won't hear of it. When Parodi leaves, Diego confesses to Setton and a fight ensues.