In a top secret Cold War military project an unstoppable, malignant organism who feeds on light and energy is unleashed threatening to destroy everything in its path.

The show takes place from 1944 to 2002 and follows the lives of three families: the Crawfords, who seek to cover up the Roswell crash and the existence of aliens; the Keys, who are subject to frequent experimentation by the aliens; and the Clarkes, who sheltered one of the surviving aliens from the crash.

Each week viewers see the gritty reality of life in a New York City Police unit as the officers go about their work with a grim determination. Two partners, Detectives Andy Sipowicz and John Kelley (later replaced by Bobby Simone), are the central characters in this weekly police drama, and personify very different approaches to their difficult job. Sipowicz's brash gruffness (covering an emotional vulnerability) is tempered by the precise and controlled demeanor of the two partners with whom he has worked.