Steel Harbinger is a shooter viewed from a 3/4 perspective, with live action full motion videos used to advance the storyline. Players take on the role of Miranda Bowen, a young woman mutated into a human/alien hybrid, who is humanity's last hope of repelling an alien invasion.

In the year 2069 North America is shaken by war. Canada and Mexico are combating the United States. Not long after the start of the war, alien pods rain down from space onto North America, sprouting metallic tentacles upon landing. The tentacles attack humans and animals, transforming them instantly into monstrous alien biomechanisms (half organic compound, half machine) bent on destroying any human life they encounter.

Dr. Bowen, a scientist, studies one of these pods in an attempt to learn about the alien species. The pod breaks out of control and attacks his daughter, Miranda Bowen (played by actress Wendi Kenya). Miranda escapes, but is still infected by the tentacles. Thus she transforms into Steel Harbinger: a half human, half biomechanical alien, and humanity's last hope for survival. Miranda must battle her way through Kansas, Las Vegas, San Francisco, Houston, Cape Canaveral, Washington, the Antarctic, an alien planetoid, and the Moon in an effort to save the Earth. Naturally, she will not make her way through these areas unopposed. She will find special weapons to destroy the aliens, and once she has killed one, it can be eaten to replenish her strength.