Parker was the chief engineer aboard the commercial towing vehicle USCSS Nostromo. He was serving on the ship during its fateful voyage in 2122, when it encountered a Xenomorph on LV-426, an incident that eventually led to the destruction of the ship and the loss of all but one of its crew, Ripley.
Parker was strongly opposed to the Nostromo landing on LV-426, citing the crew's lack of training as a good reason why they should not attempt to investigate the signal coming from the moon, but he was forced to because of the regulations. When the ship received damage when it landed, he dealt with the repairs together with Brett. When Kane returned later with a Facehugger attached to him, Parker was adamant he should just be put into stasis for return to Earth, but Science Officer Ash ignored his suggestions. Parker later witnessed the Alien´s "birth" and later witnessed the Alien dragging Brett's corpse away after killing him, becoming the first person to see the fully-grown creature and live. After Captain Dallas was also slain, Parker dismissed any suggestion of fleeing the ship and devoted all of his skills and determination to killing the creature. He later saved Ripley from Ash, whom he decapitated, revealing him to be a Synthetic. When Ash told the remaining survivors of Special Order 937 and taunted them regarding the futility of their fight against the Alien, Parker incinerated his body in a rage and ultimately agreed to set the Nostromo for self destruct and flee aboard the shuttle Narcissus with Lambert and Ripley, who were still alive then.
He was killed by the Alien later, when Peter and Lambert were ambushed while collecting a large amount of extra coolant for the shuttle's life-support system if all three were to survive while Ripley was dealing with the selfdestruct system of the ship with the intention of blowing up the ship and kill the Alien this way.