After the events of Doom II, the UAC announced that all teleporter technology was to be scrapped and their research bases were to be abandoned. However, the teleportation research as well as studies on the demonic invaders was continued on the UAC base stationed on Mars. After roughly seven months of quietly-conducted research, all contact on the UAC base on Mars was lost and, a day later, Eileen O'Hara, daughter of the scientific co-ordinator of the Mars base, sent a distress signal to the UAC's headquarters while it was still investigating the loss of contact. Following the distress signal, the Doomguy set off to Mars to take care of the Hellspawn once again, rescue any survivors, and arrive back in one piece.