Pamplona (autonomous community of Navarra's capital; north to Spain). One year later of Basajaun's crimes, a nine-months pregnant Inspector Amaia Salazar is on the city court waiting for the trial to Jason Medina, accused to rape, mutilate and kill her stepdaughter Johana Márquez. But just before of the trial, Medina is found on the court's bathroom after commit suicide cutting his veins, leaving a little note in a pocket of his jeans with just one word written: "Tarttalo". It causes that Amaia's breaks waters to birth his son Ibai, making happy her and sculptor and artist gallery James Westford, Amaia's husband. Reinstated to the active service four months later, Amaia is called by her superior to give two different cases: Monseñor Landero and Padre Sarasola requested Amaia specifically to investigate with secrecy the profanation of a little church where it was left a baby's amputated arm in the altar, and a man incarcerated in Logroño (autonomous community of La Rioja's capital) by his wife's killing has sent her a note revealing where is his wife's corpse with "Tarttalo" in the message, committing suicide after to give the note to a prison guard. Finding the corpse in Elizondo's surroundings, Amaia and Ibai back to her natal town where she meets again with tarot's card caster Aunt Engrasi, and her older sisters Flora and Rosaura, happy by the new member of the family. Discovering that the two women killed were amputated of their right arm after the crime, it matches with the church's profanation, after to learn that all arms were cut with the same pattern and weapon. Finding that it's related with the Agotes, a tribe pursued in the Middle Age by the Spanish Inquisition due to its pagan beliefs and rites, and that Tarttalo is the name of an ancient myth (a giant who eaten Christians), Amaia asks help by video-conference to her former mentor and FBI's agent Aloisius Dupree, at the same time Aunt Engrasi uses tarot to predict what could will be happen and judge Javier Markina follows with the investigation attentively. Therefore, it causes a cold in James and Amaia's relation, despite he moves Elizondo to be closer of his wife and son. However, the case makes a dramatic twist by double-entry: amputated baby's arm is revealed to be Amaia's twin sister she never knew that existed, and Amaia's mother Rosario, secluded in a mental hospital from many years ago after she tried kill Amaia at child, has killed a male nurse writing with blood "Tarttalo" under the bed, causing of re-apparition of the enigmatic and too psychiatrist Padre Saralosa, interested in Rosario and convincing Amaia to move Rosario to Saralosa's hospital university he leads. Forced to look into her family past, Amaia not only will discover the relation between her mother and a sect dedicated to a pagan cult that included human sacrifices, but that all the crimes have a too much personal connection with her. While Elizondo is in danger by an incoming flood, Amaia must solve the case and identify the killer before the most beloved for her be the next victim.