David Postlethwaite was one of the many inmates of the prison on the planet Fiorina 161 and was considered one of the more intelligent individual among inmates.
In 2179, Ellen Ripley crash landed on Fiorina 161 after the EEV ejected her along with Corporal Hicks, Newt and Bishop from the Sulaco due to the presence of the facehugger. A facehugger eventually emerged from the wreakage of the EEV and impregnated the prison dog, Spike. A Xenomorph eventually ruptured from Spike and matures to its adult form. It begins a killing spree in the prison, causing mass hysteria among the prisoners. Superintendent Andrews tried to take control of the situation, but was killed by the Alien during his "rumor control".
Ripley rallies the inmates and proposes they pour highly flammable toxic waste, which is stored at the facility, into the ventilation system and ignite it to flush out the creature. An explosion is caused by the Runner's premature intervention, resulting in several deaths. The remaining survivors, including David later form a plan to lure it into the foundry's molding facility and drown it in molten lead. He was killed by the Dog Alien when he looked through the window of a gate during the bait-and-chase style plan.