Mark Foster was a reporter who was dating Julie Herrick. He found a rival for her affections in a mysterious man named Martin Field who seemed to hold sway over the entire Herrick household. Mark was also intrigued by reports about town about an invisible man, and when Dr. Peter Drury telephoned Sir Frederick Travers that the invisible man was at his home, Mark went with Travers and his men to Drury's house but arrived too late to save the scientist.
Later, Mark discovered that "Field" was not only the invisible man, but that his real name was Robert Griffin and he was able to turn visible again by draining another person of their blood completely (which is how Drury died). Griffin lured Mark into the cellar of the Herrick house, and knocked him out, then planned to use Drury's stolen transfusion equipment to take Mark's blood. Before he could do so, however, Drury's dog Brutus burst in and attacked him, avenging his master and saving Mark.