The American destroyer escort, the USS Haynes commanded by Robert Mitchum, detects a German U-Boat, commanded by Curt Jürgens, resulting in a prolonged, deadly battle of wits.

It is 1879 in the Dakota Territories, a band of men who set out to find and recover a family of settlers that has mysteriously vanished from their home. Expecting the offenders to be a band of fierce natives, but they soon discover that the real enemy stalks them from below.