Lt. Martin Kirn (SAL LANDI), explosives expert and military instructor, has just been summoned by his commanding officer, Colonel Guardino (ROBERT CULP), and given new orders . . . straight from the Pentagon.
He is to lead a rag-tag crew of U.S. Marines on a routine bomb disposal mission to an island once used as a WWII target range.
Accompanied by Lt. Fetterman (ANDREW DIVOFF), a Military Intelligence advisor, Kirn and company land on the deserted island and find an abandoned military camp and begin searching the area for unexploded bombs. Kirn soon uncovers a bone-filled mass grave, while other members of his team hear unearthly screams and stumble upon a mysterious concrete structure.
As night falls, Kirn's suspicions are heightened as he reports the strange occurances to Lt. Fetterman, but is offered no explanation in return. In the middle of the night, Kirn is awakened by a noise outside the tent and captures an old, ragged man, the "Survivor."
While the Survivor is being interrogated, other members of the team, Hendricks and Banta, explore the concrete structure. Hendricks crawls through an opening and disappears -- his screams of terror suddenly echoing from within. Banta, reacting, tries to escape and her arm is suddenly enveloped by a white, sticky substance that burns her flesh away.
Writhing in pain, she sees an unearthly creature: the alien, XTRO. Discovering the Survivor's underground hideout, Kirn and his team find old films documenting brutal medical experiments performed in 1946 on aliens of the type seen by Banta. Suddenly it's clear to Kirn that at least one of the aliens must have survived and succeeded in exacting it's revenge on all the island's inhabitants except the Survivor.
Now Kirn's mission has become a matter of life and death as he pits himself and his team against both a vengeful XTRO and U.S. Military Intelligence, which has plotted to sacrifice them and cover up the alien's existence.
While Kirn struggles to survive and warn the world, XTRO prepares to complete it's slaughter of the Marines before escsaping in a hovering spacecraft to it's own world --perhaps to return again. XTRO - watch the skies!