A Touch of Brimstone is the twenty-first episode of the fourth series of the 1960s cult British spy-fi television series The Avengers, starring Patrick Macnee and Diana Rigg. It originally aired on ABC on 18 February 1966. The episode was directed by James Hill (known for A Study in Terror and Born Free) and written by Brian Clemens.

The episode is widely known for Diana Rigg's risqué "Queen of Sin" costume (which she designed herself), and was the most watched episode of The Avengers on its original showing. There is a publicity still of the "Queen of Sin" holding James Hill, the episode's director, on a leash.

The story is episodic, beginning with a series of pranks on fictional important international figures, an exploding cigar and a collapsing floor under a potentate's box seat in an auditorium. Mrs Peel investigates the Honorable John Cleverly Cartney, a suave, womanizing gentleman who appears to be present before, during or after these events. Lord Darcy arrives and informs Cartney that he has arranged another prank. Meanwhile Mrs Peel sneaks into Lord Darcy's place and finds a pair of fake scissors, made of rubber. But real scissors are used by an unnamed, faceless official opening the "International Friendship Club" on an electrified ribbon, killing him, and Steed and Mrs Peel can't arrive in time. The prankster did not expect to be involved in murder, and is distraught. He knows Steed and introduces him to the Hellfire Club, responsible for the pranks. Mrs Peel goes to visit Cartney and discovers he is leading the Hellfire Club, an organization based upon dressing up in old costumes and engaging in orgiastic rituals and which thrives in "ultimate sins" with the purpose to replicate the club as closely as possible.

Darcy arrives and angrily demands a special meeting with the Assembly of Superiors on the "Circle of Justice" asking why they plotted a "filthy rotten murder" and implicated him. The centre of the circle opens as a trapdoor and Darcy is killed. Steed joins the Hellfire Club and is given two tests including drinking an industrial amount of alcohol and removing a pea from the line of an axe wielding member. Steed overhears the gang in leaving and learns that they plan to stage an outrageous coup which will have the "whole country up in arms". Steed and Mrs Peel attend an orgy and investigate, spotting a cache of explosives. Steed questions a drunk girl and is told the gang are aiming to blow up Culverston House where a major international meeting is to take place. Mrs Peel enters in a "Queen of Sin" outfit holding a snake and Cartney says to the group "she's yours to do with what you will". After watching a fight, Darcy's housekeeper recognizes Steed and exposes him to the organization as a spy. He wins the ensuing sword duel against an expert. Mrs Peel goes underground and defeats one of the gang before being attacked with a whip by Cartney, who drops to his death through the circle trapdoor when his whip catches on the switch.

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