Due to a navigational error, Ripley's shuttle from the Nostromo drifts through the core systems for 57 years before a deep salvage team discovers it. She reports the deaths of her crew mates and the destruction of the Nostromo to the Company, but because there is no evidence to support her story, she is found psychologically unfit by the Company Review Board.

When the Company loses contact with the terraforming colony on LV-426, where Ripley first encountered the alien craft, she is asked to return to the planet as an advisor with a squadron of Colonial Marines and a Company representative to investigate.

A citizen of the terraforming colony on LV-426, Newt's parents are sent to investigate the alien derelict ship first discovered by the Nostromo. Her father is the first to be impregnated by an alien. Newt survives the resulting invasion of the aliens by hiding in the air duct system, using tunnels that are too small for the aliens to enter. She is later rescued by Ripley and the Colonial Marines and comes to see Ripley as a surrogate mother.

For the mission to LV-426, Cpl. Hicks serves as third in command. A serious soldier who keeps to himself, Cpl. Hicks always manages to be in the right place at a potentially lethal time, which makes him highly appreciated by his fellow Marines. When he is forced to assume command, he has the remaining members of his squadron fortify the Operations Center while they await evacuation.

Carter Burke worked for the Weyland-Yutani Corporation for nearly ten years and is the company man sent with the Marines to LV-426.

What the Marines don't know is that Burke is sent by the Bio-Weapons Division to bring back a live alien for research. He is responsible for sending the colonists to investigate the derelict spacecraft after Ripley made her report on what happened to the Nostromo. He plans to impregnate Ripley and Newt with alien embryos and sabotage the Marines' freezers on the return trip to earth, insuring that an alien would get past Quarantine.

Due to Company and military regulations, the Bishop android is sent with the Marine squadron to LV-426 to record events and act as Science Officer. Bishop obtains valuable information on the aliens' biology and behavior through his studies of the facehuggers preserved in the Medical Lab and information found in the colony logs.

When the Dropship crashes, Bishop remotely pilots a second ship to the surface from the Sulaco. Because of this, Bishop plays a key role in rescuing the last of the survivors.

PFC Hudson is nearing the end of his tour of duty when called to action on LV-426. He constantly makes cocky remarks to his fellow soldiers and anyone else who will listen. His wisecracking, swaggering style causes him to continuously butt heads with his superiors and fellow marines. He describes the other missions he has been on as a "walk in the park," but finds he is mistaken in his belief that this one will be the same.

As Commanding Officer of the Marine unit, Gorman is inexperienced, having only been in one combat drop before the mission to LV-426. Because of his poor planning of the mission and lack of knowledge of the Atmosphere Processing Station, the Marines are ordered to only use flamethrowers as use of their pulse rifles would cause an explosion. The Marines' lack of firepower subsequently causes their misfortunes in the Processing Station.

PFC Vasquez was recruited from juvenile prison, where she had been serving a life sentence. Upon joining the US Colonial Marine Corps, she is trained in the use of the M56 Smart Gun.

When ordered to use only flamethrowers in the Atmosphere Processing Station, she reluctantly hands over her ammunition and then loads a spare clip when Sgt. Apone's back is turned. She hands another to PFC Drake. When the aliens attack, she and Drake open fire, covering the Marines' retreat.

Sgt. Apone is the tough-talking, cigar-chomping team leader of the USCM squad sent to LV-426 after contact with the colony is lost. His rough, yet personal, style of command won him the respect of the soldiers under his command and the constant banter among the members of his squad shows that they are a closely knit group.

PFC Drake was recruited from juvenile prison where he had been serving a life sentence. In the Marines, he is trained to use the Smart Gun, and leads the troops through the Processing Station in their search for the aliens.

PFC Frost is in charge of collecting the ammunition from the other Marines once Lt. Gorman realizes they can't risk firing anything other than flamethrowers in the Atmosphere Processing Station. Ironically, the front of his t-shirt reads "Peace through superior firepower."

During the initial drop to the planet, Ferro remains cool and professional even when she has trouble locating the beacon. As Pilot of the Dropship, she doesn't join the others in their search of the planet and instead waits for orders to pick them up when they're ready to leave. Ferro's helmet reads, "Fly the friendly skies" and the side of her Dropship has a logo of an eagle with a gun that says, "Bug Stompers. We endanger species."

It is rumored that PFC Spunkmeyer was underage when he enlisted in the US Colonial Marine Corps. As Cpl. Ferro's Co-Pilot, he too is responsible for safely transporting the troops to the surface and then bringing them out again in one piece. He also takes care of unloading Bishop's supplies for his study of the facehuggers in the colony lab.