Dallas was the captain of the commercial towing vessel Nostromo.
Dallas, like all the crew of the Nostromo, was roused from hypersleep by MOTHER for the purposes of investigating the distress signal and (presumably) forwarding Ash his special orders (although he did not know about the order). He later called a crew meeting to tell them of the distress signal. Although he followed the order, he appeared very irritated by the detour.
Dallas was part of the landing party from the Nostromo that, upon reacting to a distress signal originating from Acheron, left the ship to investigate the source of the signal. He and Lambert held back while Kane investigated the hold of the Derelict and its cargo. He then helped Lambert carry Kane and the facehugger to the Nostromo's lift and into the airlock. He showed some concern for Kane although this may have been his own desire to enter the Nostromo and get inside from the weather. Either way Ash let the trio inside, wherin Dallas and Ash investigated the facehugger and attempted to remove it. With his irritation finally at a peak, he had the Nostromo launched before all repairs could be completed.
After Kane's chestburster was born, Dallas organized a search party that wound up having Brett captured. Unwilling to let another crewmate die, Dallas decided to try and force the Alien into the airlock via the ventilation shafts.
Dallas and the crew first attempt to capture the Alien and flush it into space via an airlock. This first attempt resulted in the capture/death of Brett in the C Deck and learning that the Alien (now fully grown) was using the air ducts to hide and move around. Dallas decided to attempt to flush it from the air ducts directly into the airlock and blow it into space. Before he went into the air ducts he accessed the central computer, revealing indirectly his nervousness about attempting to capture the Alien.
Once in the air shafts, Dallas reached the third junction and ordered Ripley to shut the air shafts behind him, preventing the Alien from coming up behind him but also blocking his escape route. He began to go down the third junction when Lambert lost the signal on the motion tracker of the Alien. Dallas discovered some slime the Alien had left behind. He began to panic slightly, and was asking if he was clear to escape. When the Alien began moving at him, assuming it was coming from above, he went down the ladder straight to the Alien. When he reach the bottom he shined the light right into the shrieking Alien's face as it attacked.
- In a deleted and later restored scene, Ripley finds Dallas cocooned in the lower sections of the ship, turning into a Xenomorph egg. Knowing what is happening to him, he begs Ripley to save him. Seeing she save Dallas from his fate, she grabbed Dallas and put him in the shuttle.