It’s happened again: three years ago, the best Spanish title in the official selection of the San Sebastián International Film Festival wasn’t a film but a series, La Mesías, directed by Javier Calvo and Javier Ambrossi, who are currently triumphing at the Spanish box office after doing so at Cannes with La bola negra. At the 74th edition of the Basque festival, the equivalent title is The Castle, a Movistar Plus Original series created by Isabel Peña (Rodrigo Sorogoyen's regular co-writer) and Eduardo Villanueva (co-writer on the aforementioned duo’s series Riot Police), one of the boldest and most surprising efforts in the line-up, even though it featured in the official section, out of competition.

Based on the non-fiction book El proxeneta (lit. "The Pimp") by actress and filmmaker Mabel Lozano (a two-time Goya Award winner in the short-film category), this series directed by Elena Martín Gimeno (Creatura, selected in the Cannes Directors’ Fortnight) and Sandra Romero (As Silence Passes By, selected in New Directors at San Sebastián) comprises six taut 50-minute episodes, making it a hefty, tough and intense proposition that is not for the faint-hearted. The Castle lays bare the inner workings of a business that is horrific, controversial and merciless: the trafficking of women for sexual exploitation. It is something that still goes on – with the backing of clients, politicians and judges – in brothels across the Spanish peninsula.

Structured around three sets of characters and starting in 1999 before concluding a few years later, the series introduces us to a young police inspector, newly assigned to the UCRIF (Central Unit for Illegal Immigration Networks and Document Forgery), and his partner, who sense that something is changing in the brothels. Elsewhere, the camera follows "The Swiss" and his associates, a gang of pimps who run sinister roadside dives where they sexually exploit girls arriving from South America and Eastern Europe. And finally, the viewer accompanies one of these women, barely 18 and with a sick son in Colombia, who has just arrived in Spain in search of money to send back home.

Skilfully blending a police thriller with personal and courtroom drama, plus a terrifying deep dive into the underworld, The Castle lays bare a dreadful reality without lapsing into prurience (the girls are never shown naked or performing sexual acts), with a steady hand and an unwavering commitment to condemning such activities. Its three narrative strands are intercut, and none of them flag, thanks to the verve shown by the directors, a script that combines pace and naturalism with chilling real-life data, and a standout ensemble wholly dedicated to the cause.

A rather stout Raúl Arévalo lurks beneath the abrasive skin of a jackal who has turned wickedness, ambition and a lack of scruples into his daily routine; Valentina Vidal breathes life into the most humane, empathetic and sensitive side of the story; and Omar Ayuso and Laia Marull play a pair of police officers willing to dive into the system’s repulsive underbelly. They are all backed up by astonishingly natural scene-stealers such as Marc Rodríguez, Rubén Martínez and Ángela Boix, among others.

The Castle is a Movistar Plus Original production staged in collaboration with Caballo Films and Mafalda Entertainment, and will premiere on the platform in Spain on 8 October.