Ensign Pavel Chekov transferred to a new assignment during the final year of the U.S.S. Enterprise's historic five-year mission (2269-70), replaced by his own navigation instructor at Starfleet Academy, Lt. Arex. By stardate 7410.2 (2271), Chekov had received a promotion to the rank of lieutenant, and had rejoined the Enterprise's crew, then under the command of Captain Willard Decker. By stardate 8130.3 (2285), Chekov had risen to the rank of full commander and served aboard the U.S.S. Reliant under the command of Captain Clark Terrell, until the escape of Khan Noonien Singh from Ceti Alpha V (2285) brought Chekov back to Kirk's crew, of which Chekov remained a key part until approximately 9524.0 (2293).