After the derelict spacecraft was discovered, the Xenomorph eggs located inside the spacecraft were taken back to Hadley's Hope for study. However, the Xenomorphs escaped and overran the colony, capturing many colonists, including Mary, and cocooning and impregnating them in the hive at the atmosphere processor.
The Colonial Marines who were sent to investigate Hadley's Hope soon found Mary in the hive, who was the only living person left. She was tended to by Corporal Dietrich, despite pleading with the Hospital Corpsman to kill her; the Marines had thus far had no experience with the Xenomorph species and were unaware of the fate that awaited Mary. She began convulsing almost immediately and was killed moments later when the Chestburster insider her erupted from her chest. Sergeant Apone and Corporal Dietrich immediately incinerated both the Chestburster, and the now-dead colonist's corpse.
Behind the scenes
To create the effect of the Chestburster bursting out of Mary's body, the actress wore a prosthetic chest piece made of foam through which the Chestburster model emerged. A replica dummy of actress Barbara Coles was then used for shots where the Marines torched her body.