The Cybernauts is the third 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 16 October 1965. The episode was directed by Sidney Hayers and written by Philip Levene.

Hammond, a middle-aged man, is attacked at home by an unseen intruder who forces his way though the door with the force of a battering ram and appears to be immune to bullets. Steed and Mrs Peel investigate. The intruder strikes again, this time a businessman named Lambert in his office, smashing his way in in the same way. On the scene, Mrs Peel notices the way Lambert's neck has been broken without bruising to the face by the angle of the head and surmises that he was killed by a type of advanced karate blow known as inku, of which there are very few expert exponents in Europe. Lambert's company, like Hammond's, is on a list of firms competing for the European rights to Japanese businessman Mr Tusamo's new circuit elements that will replace the transistor.

Mrs Peel visits a karate dojo seeking an inku specialist and is lectured by the bald sensei. After Mrs Peel proves her skill by defeating the female karate student Oyuka ("the immovable one"), the sensei allows her to join the dojo.

Steed, in place of Lambert, visits Mr Tusamo, a Japanese electronics manufacturer involved in the production of the new circuit elements to replace the transistor. Mrs Peel visits Jephcott Products, a toy factory that specializes in manufacturing electronic toys. At the karate dojo, Oyama ("the tall mountain"), a 5th dan at judo and a 4th dan at karate, demonstrates his skill to a packed room, and fits the description of the tall killer, by his height and explosive strike. Mrs Peel recognizes the man as Jephcott, the head of the toy company.

Steed visits United Automation and meets the wheelchair-bound ex-ministry scientist Dr. Clement Armstrong, owner of the factory. After Steed explains his interest in computers, Armstrong's sidekick Benson contacts the scientist via two-way videophone and mentions that someone replaced Lambert at Tusamo's office. Armstrong shows his visitor to Benson, who recognizes Steed as the false Lambert. Armstrong gives Steed a parting gift: a gadget pen containing solid ink which liquefies only in the heat of the hand, thus reducing the danger of leaks.

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