Jed Grant was hired by Doctors William Barton and Tom Morgan to be their guide in the Everglades during their hunt for the Gill Man. An extremely self-assured, amorous man, Grant became instantly infatuated with Marcia Barton, feeling that she should leave her mentally unstable husband and have an affair with him instead.
Late one night following the capture and surgical transformation of the Gill Man by Barton and his team, Jed, who was supposed to watching over the recuperating creature in the operating room aboard the Vagabondia III, left his post and tried to get fresh with Marcia. This quickly developed into attempted sexual assault, but Jed was stopped when the Gill Man escaped from the operating room and cuffed him across the midsection, knocking him unconscious.
It is unknown why exactly, but Marcia never told anyone about what Jed had tried to do to her, and thus he was welcomed (for the most part) as a guest at the Bartons' house in California. But Barton, who suspected that Marcia and Jed were having an affair, first tried to force Jed to leave. When Jed pushed the right button by muttering that Marcia hated her husband, an enraged Barton killed Jed by clubbing him to death with a pistol.