Commodore Matt Decker was a Human Starfleet flag officer assigned as the commander of the USS Constellation.
In 2267, the Constellation was on a routine survey mission near system L-374 when it encountered an alien planet killer systematically destroying the planets of the system.
During the encounter, his ship became hopelessly damaged, rendering all power plants dead. Decker remained on board to confront the machine, beaming his entire crew to what he believed was safety on the third planet of the L-374 system. When the alien machine destroyed that planet, and further damaged the Constellation, Decker was the only survivor.
Decker was later discovered by the USS Enterprise, who transported him from his broken ship. He initially resisted the effort to do so, stating that he had never lost a command before. Consumed with grief over his actions, and with an almost reckless desire for revenge, Decker seized command of the Enterprise after losing contact with the Constellation, leaving Captain Kirk stranded aboard the Constellation.
He then launched an attack on the planet killer, a futile move which resulted in severe damage to the Enterprise. When communications were restored with the Constellation, Captain Kirk gave Commander Spock a direct order to relieve Decker on his personal authority as Captain of the Enterprise. Decker submitted to Spock's relieving him of command after being threatened with arrest if he did not comply. Decker subsequently stole an Enterprise shuttlecraft, piloting it into the maw of the creature in an attempt to destroy it.
Decker was killed in the attempt. While the attack failed to cause significant damage - the Planet Killer had lost a small amount of power - it did demonstrate to the Enterprise crew how to disable the creature. Kirk drove the Constellation into the maw of the Planet Killer, which died when it was unable to handle the energy released by the Constellation's exploding impulse engines. Following the battle, Captain Kirk noted in his log that Decker gave his life in the line of duty. (TOS: "The Doomsday Machine")
Background
The character of Matt Decker was devised by writer Norman Spinrad. The producers of Star Trek: The Original Series initially asked him to write a part for Robert Ryan. Intending to take inspiration for the plot of "The Doomsday Machine" from the story of Moby Dick, Spinrad developed the role as being similar to Captain Ahab. The character was intentionally made less intense after actor William Windom was instead cast as Matt Decker. Stated Spinrad, "Things had to be adjusted. I had to make him a little softer, and I think it might have taken some of the edge off of the story. In the original version, Commodore Decker was much stronger. They don't find him slumped over in the ruined ship. Instead, they find him staring out the viewscreen and in a very bad mood. There was also the feeling that a guest star with that kind of presence would overshadow Captain Kirk, and therefore his character had to be toned down and his lines reduced." (Captains' Logs: The Unauthorized Complete Trek Voyages, p. 48) Likewise, in most of the preliminary drafts of the episode's teleplay, Decker did not sacrifice himself, but instead survived to admit his mistakes and voluntarily retire. The core of this scene was later recycled into the ending of "The Deadly Years", where Commodore Stocker admits to Kirk that his taking command of the Enterprise was in the wrong. (citation needed • edit)
William Windom once said, at a public appearance, that he patterned his portrayal of the character after Humphrey Bogart's Captain Queeg from The Caine Mutiny, particularly the obsessive-compulsive habit of toying with objects in his hands. (citation needed • edit)
In an excerpt from the writer's guide to Star Trek: The Motion Picture featured in The Making of Star Trek: The Motion Picture, Susan Sackett and Gene Roddenberry's book about the movie, Matt Decker is established as the father of Will Decker. Several other members of the Decker family are featured in the Starfleet Academy comic book series.
In James Blish's adaptation of "The Doomsday Machine", Decker's first name is "Brand" and, in common with the episode's majority of earlier script drafts, he doesn't pilot the shuttlecraft into the planet-killer. In addition, after Decker orders McCoy off the bridge with the statement, "Mr. Spock knows his duties under regulations, doctor... do you?" Blish gives McCoy one final retort: "Yes, sir – go to sickbay and wait for the casualties you're about to send me."