Amaia Salazar returns to the Baztán Valley to investigate suicides that have a mysterious and jarring pattern.

Salazar, who is pregnant and about to give birth to a baby boy, returns as the detective from The Invisible Guardian and leads the investigation.

At the City Court, Amaia Salazar and her team await the trial of Jason Medina who has been accused of the rape, mutilation and murder of his stepdaughter, Johana Márquez.

Just before the trial starts, Medina is found dead in the court's bathroom having committed suicide by slashing his wrists, and leaving a little note in a pocket of his pants with just one word written: "Tartalo".

First, Monseñor Landero and Padre Sarasola requested Amaia specifically to investigate with secrecy the profanation of a little church where a baby's amputated arm was found in the altar and second, a man incarcerated in Logroño for the murder of his wife, calls for Amaia with the message that he will reveal only to her the location of his wife's corpse.

She finds that "Tartalo" refers to an old pagan legend told by the Spanish Inquisition where a giant eats Christian babies to stop them from growing up religious.

In the meantime, Amaia and her family move to Elizondo after her Aunt Engrasi mentions the abandoned house that her grandmother left behind.

Amaia's constant calls to work cause a rift in her relationship with James, despite him moving to Elizondo in hopes of growing the family.

Forced to look into her family's past, Amaia discovers the relation between her mother and a sect dedicated to a pagan cult that included human sacrifices, and that all the crimes are connected to her.