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."[6] 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.

[7] Leonard Maltin gave the film a negative review, panning the film's production values.[8] 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".[9] 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.

[2] 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.[10] 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.[11]The film has developed a cult following over the years since its release, and is now considered a cult classic.[12]