Although the movie can be categorized in the horror genre, it differs from most other horror movies by the deeper existentialist message.
Like the title suggests, the movie is about a man who is the last remaining human survivor on earth. Robert Morgan (played by Vincent Price) is a scientist who is the only survivor of a deadly virus that turned all other humans into zombies and vampires.
Morgan is immune to the virus and tries to kill the horrifying creatures during daytime, when they are weaker. At night the zombies and vampires grow stronger, and Morgan has to lock himself in his house to prevent them from getting to him.