The central twist of Aliens is the revelation of the sheer scale of the Xenomorph infestation and the true, nefarious motives of Carter Burke. Initially, the mission is presented as an investigation into a loss of communication. The marines, and the audience, are unprepared for the discovery that all 158 colonists on LV-426 have been cocooned to serve as hosts for a massive alien hive, ruled by a colossal Alien Queen. This transforms the mission from a potential rescue to a desperate fight for survival against an army of aliens.
The film's most significant betrayal comes from Carter Burke. He is unmasked as a villain who deliberately sent the colonists to investigate the derelict ship Ripley had warned them about, hoping to profit from any resulting alien specimens. His villainy escalates when he attempts to smuggle facehugger specimens back to Earth by impregnating Ripley and Newt while they sleep. He disables Ripley's camera feed and releases the facehuggers in the medlab with them, a plot twist that fully exposes the depths of his corporate-driven depravity. Burke ultimately meets his end when he is cornered and presumably killed by a Xenomorph after the power is cut. A deleted scene, however, shows Ripley finding him cocooned in the hive, where she hands him a grenade to end his own life before a chestburster can emerge.
The film's ending features another major surprise. After Ripley heroically rescues Newt and seemingly destroys the alien hive and the Queen in a massive thermonuclear explosion, they escape to the Sulaco. However, the Queen is revealed to have stowed away on their dropship. This leads to a final, climactic battle aboard the Sulaco where Bishop is torn in half and Ripley, using the power loader, finally expels the Queen into the vacuum of space. The film concludes with the apparent survivors—Ripley, Newt, an injured Hicks, and the bisected but still-functional Bishop—entering cryosleep, creating a powerful image of a newly formed family finding peace. This peaceful ending is tragically and controversially undone in the opening moments of the sequel, Alien 3.