"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 until the intruder was dealt with, 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)
"Admiral, this is an almost totally new Enterprise. You don't know her a tenth as well as I do."
- - Willard Decker to James Kirk, on the new Enterprise. (Star Trek: The Motion Picture)
"No, sir. I don't think you're sorry. Not one damn bit. I remember when you recommended me for this command. You told me how envious you were, and how you hoped you'd be given a starship command again. Well sir, it looks like you found a way."
"Report to the bridge, commander. Immediately."
- - James T. Kirk and Willard Decker on Kirk taking command of the Enterprise. (Star Trek: The Motion Picture)
"Captain, as your exec, it's my duty to point out alternatives."
- - Willard Decker to James T. Kirk on his duty as first officer. (Star Trek: The Motion Picture)
"Permission to speak freely, sir?"
"Sir, you haven't logged a single star hour in two and a half years. That, plus your unfamiliarity with the ship's redesign, in my opinion, sir – seriously jeopardizes this mission."
- - Willard Decker confronting James T. Kirk. (Star Trek: The Motion Picture)
"Moving into that cloud, at this time, is an unwarranted gamble."
"How do you define unwarranted?"
"You asked my opinion, sir."
- - Willard Decker and James T. Kirk on whether or not to go into V'ger. (Star Trek: The Motion Picture)
"This is how I define unwarranted!"
- - Willard Decker to James Kirk, after V'ger vaporizes Ilia. (Star Trek: The Motion Picture)
"Jim, I want this! As much as you wanted the Enterprise, I want this!"
- - Willard Decker, before joining up with V'ger. (Star Trek: The Motion Picture)
Will Decker was played by actor Stephen Collins.
Despite Will Decker having the same surname as Commodore Matt Decker, 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. 
Stephen Collins felt that an important part of why he was cast in the role of Will Decker 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)
For the entry about Williard Decker (Will Decker) in the Star Trek Encyclopedia (3rd ed., p. 106), he is identified as the son of Matt Decker. In a background note for this entry, the authors state that Paramount publicity materials for the first movie suggested the existence of this relationship.
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)
The backstory regarding Will Decker's relationship with Lt. Ilia was "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 TNG Season 1.
In The Sorrows of Empire, his mirror universe counterpart was depicted as being the first officer of the ISS Enterprise from 2270 to 2277. After a failed attempt to assassinate Spock on Deneva at the behest of his father Grand Admiral Decker, he was killed using the Tantalus field. Grand Admiral Decker swore vengeance against Spock but he was killed himself shortly afterwards.