After exposure to radiation, an atomic research scientist finds himself changing into a murderous, lizard-like creature every time he is exposed to sunlight.
I remember this film as a kid. It's one of those really bad films that you kind of, sort of, enjoy watching when no one is looking:
Bob Stephens of the San Francisco Chronicle wrote, "But I must confess that I enjoy Demon. Its naivet is a more reliable pathway to wonder than the cynicism and condescension of contemporary fantasy films could ever be." TV Guide gave the film a negative review, awarding it 1.5 out of 4 stars and calling it "laughable", but also commented that the monster costume was good.
 Leonard Maltin gave the film a negative review, panning the film's production values. In his book The Encyclopedia of Monsters, author Jeff Rovin called it "a clever twist on the Wolfman theme" and an "effective and gritty film [that] boasts an excellent monster costume". Allmovie gave the film a positive review, calling it "a staple of TV horror programming since the early 1960s", praising the film's claustrophobic feel, editing and actor/director Clarke's performance as the lead character, while criticizing the film's stock characters and "clunky" dialogue.
 In Cult Horror Films, Welch D. Everman wrote that the film expresses traditional 1950s themes: a warning about the dangers of nonconformity and a mixed message about nuclear energy. Chris Barsanti wrote in The Sci-Fi Movie Guide that the film distinguishes itself from other 1950s radioactive monster films by being an allegory for alcoholism.The film has developed a cult following over the years since its release, and is now considered a cult classic.