Set in Paris, the live-action television series Spiral follows the men and women who operate within the French judicial system, from the police officers on the street to the lawyers and judges in the Palais de Justice. The drama focuses on the relentless and often morally ambiguous pursuit of justice, where personal and professional lines constantly blur as the characters navigate cases that reveal the darkest aspects of humanity.

At the center of the series is Police Captain Laure Berthaud, a skilled but impulsive detective who leads a plainclothes CID squad from a territorial division. Devoted to her work to the point of self-destruction, Berthaud is known for her tenacity and her willingness to use borderline methods to close cases, often putting her at odds with her superiors. Her team includes her closest lieutenant, Gilles "Gilou" Escoffier, a formidable and brutal enforcer whose own struggles with drugs and his blurred line between cop and criminal create constant tension. The more responsible and by-the-book Frédéric "Tintin" Fromentin serves as the squad's stable presence, often disapproving of his colleagues' illegal tactics but remaining loyal.

The legal arena is represented by a trio of compelling figures. Judge François Roban is an experienced and solitary investigating magistrate who is coldly committed to uncovering the truth, even as he faces political pressure and a deteriorating personal life. Assistant Prosecutor Pierre Clément is a young, principled, and somewhat naive attorney who believes in the integrity of the justice system, a stance that frequently brings him into conflict with his cynical superiors. In contrast, lawyer Joséphine Karlsson is a brilliant and ruthlessly ambitious defense attorney, amoral and always seeking cases that will bring her fame and money, even if it means associating with violent criminals.

The narrative arcs are structured as season-long investigations that tackle grisly cases such as serial killers, sex trafficking, terrorism, and organized crime. The series begins with the discovery of a young Romanian woman's body on a rubbish dump, an investigation that uncovers a sprawling network of corruption. Subsequent seasons involve a serial killer known as the Butcher of La Villette, a political scandal involving a mayor and a bribery cover-up, and a group of extremists waging war on the Parisian gendarmerie. A notable arc in a later season sees Laure and Gilou investigating the death of an undocumented Moroccan child found in a laundromat, a case that exposes the exploitation of homeless migrants by organized crime. As the series progresses, the personal lives of the characters spiral out of control. Laure's inability to build lasting relationships and her abandonment issues are a recurring theme, while Gilou ends up in prison before being forced to go undercover with a violent criminal gang. Judge Roban eventually faces forced retirement, and a younger, less experienced judge, Lucie Bourdieu, takes his place, creating new frictions. The final season concludes with Gilou infiltrated into a ruthless gang while Laure's team is sidelined, only for their investigations to intersect with a case being defended by Joséphine, forcing them to reunite for a tense and dangerous climax.