Baron Frankenstein's granddaughter turns the legendary outlaw's partner into a monster.

While escaping from a posse, Jesse James and Hank Tracy, a wounded member of his gang, seek help from Juanita, a young Mexican who leads them to an ancient mission ruin inhabited by the grandchildren of the infamous Dr. Frankenstein. After sending Jesse away, ostensibly to secure medicine, Frankenstein's granddaughter Maria performs a brain transplant on Hank and transforms him into Igor, a monster similar to the one created by her grandfather. Jesse returns to the mission, and the monster, no longer recognizing his friend, knocks him unconscious; Maria then prepares to perform a similar operation on Jesse. Juanita arrives with Marshal McFee, however, and both Igor and Maria are killed. Jesse is led away by the marshal, and Juanita promises to wait for his return.