"That's why you're staying aboard."
At some point in his Starfleet career, Decker held a posting on the planet Delta IV, where he entered into a romantic relationship with the Deltan Ilia. Decker ultimately chose to end the relationship, however, subsequently leaving Delta IV without saying goodbye to Ilia.
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 the 2270s, 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." (Star Trek: The Motion Picture)
Commander Willard Decker was to have featured as the Enterprise's first officer in Star Trek: Phase II. Yet, the story of Willard Decker and Lt. Ilia were "borrowed" for the characters of William T. Riker and Deanna Troi when Star Trek: The Next Generation went into production. Riker duplicated many of Decker's character traits, as well, at least in the first season.
It was clearly established in the novelization of the film that Will Decker was the son of Commodore Matt Decker. However, there is no on-screen evidence indicating any familial relationship between them. Production information on this topic is sparse, at best. The reference guide Star Trek Phase II: The Lost Series does state that Will Decker descends from numerous other Starfleet officers, some of whom are of flag rank. It is possible that Commodore Matt Decker could have been one such flag officer; indeed, a footnote in The Making of Star Trek: The Motion Picture identifies Will Decker's father as "Commodore Matt Decker of the Star Trek television episode 'The Doomsday Machine'." Further support seems to exist on StarTrek.com. The official Star Trek website clearly states that Captain Decker is the son of Commodore Matt Decker. 
Will Decker was played by actor Stephen Collins. He felt that an important part of why he was cast in the role was that, unlike many or all of the other candidates for the part, he did not revere Star Trek. "I think it really helped me get the part," he said, "because I walked into the meeting and, you know, it was just a part to me. I mean, I was much more interested in meeting [director] Robert Wise than I was at the thought of doing Star Trek." Collins auditioned for Wise and, by the time he returned home, not only had Wise decided that Collins was the best applicant for the role but an offer had also been struck up for him to portray the character. (A Bold New Enterprise - The Making of Star Trek: The Motion Picture, Star Trek: The Motion Picture (The Director's Edition) DVD special features)
While playing Will Decker, Stephen Collins often felt as if he was somewhat intruding on the bubble-like atmosphere of the main cast, virtually all of whom had been in Star Trek: The Original Series (with the exception of Persis Khambatta as Ilia). "Some of that really helped in terms of the part I played," Collins mused, "because I was supposed to be this outsider who they had to have along on the trip." (A Bold New Enterprise - The Making of Star Trek: The Motion Picture, Star Trek: The Motion Picture (The Director's Edition) DVD special features)