Once the delegation arrived aboard the USS Enterprise for transport to Babel, Thelev's mission was to destroy the Enterprise and instigate mutual suspicions between the various Federation members and lead to interplanetary war, as part of a larger Orion plot to allow continued raids on Coridan III, and the suppling of dilithium to all sides.
Thelev successfully murdered the Tellarite ambassador, Gav, but was unsuccessful in his attempt to murder Captain James T. Kirk. During the fight, the Enterprise crew discovered Thelev's true identity and the secret communications device hidden in his fake antennae which signaled the Enterprise's position to an Orion vessel. When the Enterprise successfully dispatched the Orion vessel, Thelev committed suicide. (TOS episode: "Journey to Babel")