Written by Oriol Paulo and Guillem Clua with the collaboration of Lara Sendim, the screenplay is an adaptation of the 1979 novel of the same name by Torcuato Luca de Tena.

The plot follows Alice Gould (Lennie), a woman entering a psychiatric ward in order to investigate the mysterious circumstances pertaining to a death in the facility.

Dr. Castell introduces the facility to Alice, who starts doing investigative fieldwork on the death of Dr. Raimundo García del Olmo's son, Damián.

Alice personally meets Director Samuel Alvar, claiming that she has already exchanged letters with him upon guidance from García del Olmo so she could know how to enter the institution.

She claims that she tricked Ruipérez and feigned a mental illness to enter the facility, telling how she met García del Olmo and set out an investigation to determine what happened to Damián.

Urquieta finds one of the twins dead in a cell, making it evident that the events of the secondary timeline pertain to the aftermath of Alice's plan.

[8][9] Production began when Atresmedia Cine and Warner Bros., who had the rights to the novel, approached Paulo in June 2019, when he was about to start filming The Innocent.

[12] His approval of their offer came six months later after rereading the novel and developing fascination with the main character, Alice Gould, whom he described as an "upper-class woman, so brutal, so intelligent and with such a verbal argumentative ability.

[8] Paulo stated that while rereading the novel, he visualised Lennie in the role of Alice, whom he found having many qualities that the character has, among which are intelligence and mental agility, and being the only suitable actress to play it.

[30] Rubén Romero Santos of Cinemanía rated the film 3½ out of 5 stars, deeming it to be "a thrilling and elegant" exercise of narrative shell gaming.

[31] Beatriz Martínez of El Periódico de Catalunya rated the film 4 out of 5 stars, considering that "the intrigue finds a perfect balance between stylization and magnetism" and also praising the "titanic" adaptation work, "in which not only the language is cleaned and distilled, but also the stale substratum from the novel with a touch of pop aestheticism.

"[32] Raquel Hernández Luján of HobbyConsolas rated the film with 50 out of 100 points ('so-so'), considering that it fails at respecting the heart of the novel (the dignification of the mentally ill and the value put on the work of the staff), praising the art direction and costume design while negatively assessing the direction of actors and the overextended footage achieved by a number of consecutive closures.

[34] Paula Vázquez Prieto of La Nación concluded, "With modest ambitions and a clear goal of entertainment, [the film] fits into the narrative without much originality but with the necessary rhythm to embrace all its pieces in each new turn that the revelations offer.

[39] Reviewing for El Mundo, Luis Martínez wrote that the film "ignores the real and almost forced possibility of turning the plot into a metaphor or the text into a context", concluding that it is "an obsessive and delirious thriller.

"[40] According to Vázquez Prieto, the film fails to give a surprising plot twist after "the 'crazy' adventures of [the lead actors'] characters on their backs".

[35] Marín Bellón applauded Lennie for showing her versatility as an actress and delivering a flawless performance, noting the mysterious atmosphere that she brought to her role.

[31] Similarly, Avinash Ramachandran of The New Indian Express opined that Lennie "owns the screen" and the other actors are "stunning in equal measures.

[9] Pando positively compared Lennie's portrayal of Alice to those of Peter Breck's reporter Johnny Barrett in Shock Corridor and Leonardo DiCaprio's policeman Edward Daniels in Shutter Island (2010).