Willard Decker was a 23rd century Starfleet officer who, in 2270, was assigned as captain of the USS Enterprise subsequent to James T. Kirk's promotion to admiral.
Decker was recommended by James T. Kirk to command the refitted USS Enterprise following the completion of Kirk's legendary five-year mission and subsequent promotion to rear admiral in 2270. Despite his recommendation, Kirk conveyed his feelings of envy towards Decker, telling the younger officer his hopes of commanding a starship again in the future. The now-Captain Decker would go on to oversee the extensive refit of the Enterprise throughout its entire eighteen months.
In 2273, when it became known that an extremely powerful force was en route to Earth, Kirk seized upon the opportunity to convince Admiral Nogura to grant him command of the Enterprise in order to intercept the threat. Kirk temporarily downgraded Decker's position to executive officer and his rank from captain to commander, something Decker protested. Decker bluntly expressed his belief that Kirk was only using the emergency to retake command of the Enterprise, and that his inexperience with the newly upgraded vessel could jeopardize the mission. Regardless, Kirk remained adamant that his own experience in dealing with unknown threats was pivotal to their mission, and maintained command. Later, after the unexpected death of Commander Sonak in a transporter accident, Decker was also forced to double as science officer. This assignment proved only temporary, however, when Spock arrived to offer his services.
Prior to the Enterprise's launch, Decker received the surprising but pleasant news that his former love, Ilia, now a lieutenant, had arrived aboard the Enterprise to serve as navigator. When they were alone, Decker apologized to Ilia for leaving her, but Ilia made it clear that she was still harboring negative feelings regarding the unresolved manner in which their relationship ended. They had little time to rekindle their relationship, however, as Ilia was taken from the bridge by a probe from the enemy vessel, reduced to a data stream, and replaced by a probe that duplicated the Deltan officer's form.
When it was found that the probe not only copied Ilia's appearance but her memories as well, Decker was given the assignment of reviving Ilia's memories, in an attempt to make contact with her and learn more about the entity approaching Earth, whom the crew had learned called itself "V'ger." His efforts proved to be a partial success, as, for a moment, Ilia's personality was able to surface and explain to Decker that V'Ger was seeking an unknown "creator" on Earth.
Upon arriving at Earth, V'Ger threatened to destroy the planet unless it was allowed to meet with the creator. Decker was among the landing party which also included Kirk, Spock, and Doctor McCoy, which discovered that V'Ger was actually an old Earth space probe known as Voyager 6 and was seeking to evolve by joining with its creator, namely, a human. Having lost the captain's seat and no longer having Ilia, Decker volunteered to physically join with V'Ger. Decker and the Ilia probe disappeared into a blinding source of light, evolving into a new lifeform, leaving for parts unknown.
Upon returning to the Enterprise, Kirk issued orders that both Decker and Ilia be officially listed as "missing."