A wave of practical jokes is damaging Britain's international standing and when a VIP is killed by one, the Avengers step up their hunt for the perpetrators.
Mrs Peel discovers that Cartney, the chief suspect, runs a recreation of a Regency Hellfire Club and she witnesses an angry member challenge Cartney over the killing; the next day, his body is fished from the river. Steed and Emma attend a debauched orgy at the Hellfire Club and notice a sedan chair filled with dynamite heading for the basement which is linked by tunnels to Parliament. Emma is drugged and dressed as a dominatrix, in corset and spiked collar, while Steed realises they have discovered a modern Guy Fawkes consipracy and duels with the club's leading swordsman
Cigars exploding, whoopee cushions, faulty chairs, spiders in ones soup - all used on foreign leaders. Humiliated. Only the Avengers tackle this and make the incidents sound like life and death. Well, one of them does result in death. There's a sharp performance by Peter Wyngarde - aka Jason King - as the leader of a Hellfire club, the one behind these incidents. He's quite a dynamic character and a nasty one too. The removing the pea before the axe strikes is quite tense - that's Steed initiation into the club. Emma Peel gets drugged and is dressed in a dominitrix suit- she looks quite fetching. It's a brilliant episode with an air of menace in the air - it ends with a rousing sword fight between Steed and one of the Hellfire's goons who has a steel claw behind his glove and Mrs Peel goes up against the leader who uses a whip but it's a crack of a whip too far for him as he makes a mistake and gets his up commence.