Kryton was an Elasian male of a noble family and the bodyguard of Elaan, the Dohlman of Elas. Kryton loved Elaan and wished to marry her. He was furious when he heard of her wedding plans with the ruler of Troyius, leading him to sell out to the Klingons because of jealousy.
Following her arrival, he stood guard outside of the Dohlman's quarters, instructed to prevent or allow only certain individuals to enter her cabin.
Mid-journey, Kryton entered Main Engineering and rigged the antimatter pods to blow up when the warp drive was engaged. Before he was finished, he was caught in the act by Engineer Watson, whom he immediately killed by snapping his neck. He finished his work and contacted the Klingon warship that was paralleling the Enterprise. The plan was for the warship to taunt the Enterprise into engaging her warp drive to destroy itself and help ensure a Klingon foothold in the system.
When his sabotage was discovered Kryton mocked Captain Kirk's attempt to question his motives. After being threatened with the prospect of a mind meld he stole a phaser from Lieutenant Evans and killed himself. (TOS: "Elaan of Troyius")