Ash (Ian Holm) is the Nostromo's inscrutable science officer. He administers medical treatment, conducts biological research and is responsible for investigating any alien life forms the crew may encounter. It is at Ash's insistence that the crew investigates the mysterious signal emanating from LV-426. Ripley becomes suspicious of him when he breaks quarantine protocol by allowing Kane, Dallas, and Lambert to re-enter the Nostromo while the Alien facehugger is attached to Kane. Captain Dallas later informs Ripley that Ash had abruptly replaced the ship's previous science officer, whom Dallas had done six previous missions with, just as the Nostromo left Thedus for its return journey to Earth. Over Ripley's objections, Dallas entrusts Ash with all science-related decisions.
After the Alien has killed Kane, Brett and Dallas, Ripley discovers through the ship's computer "MOTHER" that Ash has been given special orders by the Weyland-Yutani Corporation to ensure the return of the Alien to the company's laboratories, even at the expense of the crew. When confronted by Ripley, Ash attacks her. He is hit in the head by Parker, and the force almost completely severs his head. It is at this point that Ash is revealed to be an android. After suggesting that the crew will not survive the Alien's attacks and giving them his sympathies, Ash is destroyed by Parker using a flamethrower.