Dr. Gene Tuskin works with troubled children, perhaps none more troubled than Regan MacNeil, who suffers from bad dreams and repressed memories. The memories she represses are of the time she was possessed by a demon. Dr. Tuskin's invention, a device that hypnotizes two persons and links their minds together, reveals that the demon, named Pazuzu, still lurks within her. It is desperate to emerge again and wreak havoc. Meanwhile, Father Philip Lamont is ordered by his cardinal to investigate the death of Father Merrin, the priest who died while performing an exorcism on Regan. Father Lamont undertakes his task reluctantly. He feels unworthy of his assignment. He also feels that Evil is literally an entity and that this entity is winning the battle over Good. His investigation takes him to Africa where he locates another recipient of Merrin's exorcising and learns something fascinating and terrible about locusts.

This movie is a stark portrayal of life among a group of heroin addicts who hang out in "Needle Park" in New York City. Played against this setting is a low-key love story between Bobby, a young addict and small-time hustler, and Helen, a homeless girl who finds in her relationship with Bobby the stability she craves. She becomes addicted too, and life goes downhill for them both as their addiction deepens, eventually leading to a series of betrayals. But, in spite of it all, the relationship between Bobby and Helen endures.