In 2268, Petri was assigned to conduct the Elasian Dohlman Elaan to Troyius for an arranged marriage to the leader of Troyius. Despite his antipathy for the Elasians, Petri hoped that the marriage would end the long war between the two worlds. However, Elas's Council of Nobles and the Troyius Tribunal both feared that Elaan's arrogant and demanding nature would prove unacceptable to the Troyian people, and Petri was ordered to make her "acquainted" with genteel Troyian customs.
While in transit to Troyius aboard the USS Enterprise, there were a number of arguments between Elaan and Petri, and eventually Petri was stabbed in the back by the Dohlman. Petri survived, but he refused to continue teaching Elaan, so Captain James T. Kirk assumed his role. (TOS episode: "Elaan of Troyius")