"There is a clause in the contract which specifically states: Any systematized transmision indicating a possible intelligent origin must be investigated."

After landing on a barren planet, Warrant Officer Ripley works to decipher the transmission and discovers that the signal is actually a warning, not an SOS. But it is too late to turn back as three members of the crew have already left to investigate the derelict ship.

"I can't see a goddamned thing!"

"You still don't know what you're dealing with, do you? Perfect organism. Its structural perfection is matched only by its hostility"

"I admire its purity, a survivor; unclouded by conscience, remorse or delusions of morality."

Warrant Officer Ripley ultimately finds that the alien must be destroyed, not only to save herself and her crew, but all of mankind. She sets the Nostromo to self-destruct and prepares to escape in a shuttle, leading to the final explosive battle for survival.

"Final report--The Commercial Starship Nostromo. Third officer reporting. The other members of the crew, Kane, Lambert, Parker, Brett, Ash, and captain Dallas, are dead. Cargo and ship destroyed. I should reach the frontier in about six weeks. With a little luck, the network will pick me up. This is Ripley, last survivor of the Nostromo, signing off."