Members of an exclusive karate club put theory into practice and use men for guinea pigs to prove their skill. Ralph Haig is their latest victim, but when he successfully fights back against their individual attacks, the club members team up and batter him to death.
Steed and Mrs Peel are required to find the murderers. They decide to visit the judo clubs, and Mrs Peel visits The Atarashii Club, where she is told about George Moran, who was thrown out for vicious and irresponsible behaviour, but has now joined the mysterious K club.
Steed puts a sting into operation. He visits a bar used by members of the K club and, using his friend Jack Day as an accomplice, he paints a picture of himself as an expert in Kung Fu. Martin Glade of the K club takes the bait and, a couple of days later, he approaches Steed in the street and invites him to visit the club. Steed makes more than a good account of himself against K club member George Moran, who overreacts and attempts a death move, something which the other club members are shocked at as they dont know if Steed has told anybody else he would be visiting the club. Luckily for them Steed counters and throws the errant member. Glade interrupts and the bout is ended. He suggests that next time Steed should fight him.
Steed confides to Mrs Peel that there is something peculiar about the K club. He asks Mrs Peel to start shadowing him because he believes that the club are about to kidnap him. He is, of course, right. Several days later he is accosted by Moran and forced into a Jag. Mrs Peel sees Steed adjusting his bowler the signal for trouble. Steed is taken back to the K club. Glade welcomes him and suggests to Steed that they continue the bout of a few days earlier. Steed declines, but Moran jumps in impatiently and chops Steed to the adams apple. Glade pulls him back and insists that they stick to procedure. Steed realises that he is in a fight for his life and rushes at Glade. The fight begins and Steed is giving a good account of himself against the horde.
Outside the club, Mrs Peel is getting impatient and decides to enter to see what is happening. She is confronted by a man dressed for judo who refuses to let her in. She brushes him to one side and, entering the gym, joins the affray. With her help, Steed is able to subdue the killers and, as he attempts to escape, Mrs Peel downs Glade with a thrown statuette. The Avengers are victorious and the vicious K Klub is Klosed.
Observation this is essentially the same story as The Mohocks, but without the embellishments that story had via the social backgrounds of the villains. Here, the villains are less well-fleshed out, and much of the story is taken up with meaningless fights.