Dennis Parker is the Chief Engineer aboard the commercial towing vessel Nostromo.
Parker was close friends with fellow engineer Brett. Although they were both competent at their jobs, they frequently found themselves in disagreement with the ship's captain, Dallas. Specifically, Parker disliked the fact that Brett and he received fewer shares than what the rest of the crew was earning. Parker also found himself frequently at odds with Warrant Officer Ellen Ripley.
In the year 2122, the Nostromo was returning from a job in the Solomons and passed through the Zeta II Reticuli system. Due to a command program in the ship's computer, the crew of the Nostromo came out of hypersleep early in response to the interception of a distress beacon. Upon learning that the ship had been re-routed, Captain Dallas alerted the crew that they would be following the beacon to its source to investigate it. Parker complained about the situation and demanded full shares for his work.
The crew took a drop-ship down to the surface of a planetoid (which later became known as Acheron), but the intense weather patterns caused a great deal of damage to the ship. When asked about how long it would take to affect repairs, Parker hedged the estimate and told Dallas that it would take twenty-five hours to fix everything.
While Parker and Brett were manning the repairs, the away team found an alien entity that attached itself to the face of the first officer, Kane. Despite quarantine procedures, Kane was brought back onto the ship and placed in the infirmary. Once the repairs were completed, the crew and the alien "facehugger" were brought back to the Nostromo. The entity impregnated Kane with its seed and used him as a host body to give birth to a larger, more aggressive alien. This alien, often referred to as a Xenomorph, grew to a size of more than seven feet in height and began systematically killing everyone on the ship.
Following the death of Kane and then Captain Dallas, Warrant Officer Ripley took control of the ship. She accessed the ship's computer and discovered a horrifying truth. The company they worked for, the Weyland-Yutani Corporation had implanted an android spy aboard their ship masquerading as their science officer. This spy, Ash, was under orders to recover the alien organism and return it to Earth, even at the expense of the crew. After being exposed as a traitor, Ash tried to kill Ripley. Parker came upon them and tried to restrain Ash, but the android was more powerful than him and shirked him off. Parker clubbed Ash across the head repeatedly, practically knocking it off. It was at this point that he learned that Ash was a synthetic. The group interrogated Ash, after which, Parker incinerated his robotic remains.
Although the problems with Ash had been dealt with, they still had to contend with the Xenomorph still loose aboard the ship. Ripley decided that they should take their chances with the shuttle and activate the Nostromo's self-destruct sequence. Parker and Lambert went down to the maintenance bay to collect coolant canisters and came across the alien. The creature lunged at Lambert and Parker tried to save her, but the alien swatted him to the side with its spiny tail. It then turned towards Parker and killed him by spearing him with its secondary jaw. Parker's remains were destroyed when Ripley set off the self-destruct sequence.