Marcia Barton was the wife of Dr. William Barton. An extremely rebellious and free-spirited young woman, Marcia would hunt and go on daring adventures in order to escape the tedium of her unhappy marriage to the increasingly unbalanced William. When William and his research team set sail from their home in California for the Florida Everglades aboard the Vagabondia III to hunt for the Gill Man, Marcia went along. Her gung-go attitude attracted the unwanted attention of the expedition's young guide, Jed Grant. Regardless of her dislike of her husband, Marcia was entirely uninterested in Jed, and told him so. This didn't stop him from making numerous passes at her, nor did it stop William from becoming paranoid about the two.
During the remainder of the voyage, Marcia vented her frustration by shooting at sharks from the Vagabondia's deck, and later she accompanied Jed and Dr. Tom Morgan on their initial dive to look for the Gill Man, despite William's fierce objections. During the dive, Marcia swam too deep and was overcome with the "raptures of the deep," temporarily losing her mind and insisting on removing all her scuba gear. This forced Jed and Tom to abandon their hunt for the Gill Man to swim back and save her.
After the Gill Man's eventual capture and surgical transformation by William, Tom and their colleagues Dr. Borg and Dr. Johnson, Marcia mistakenly believe that after such a success her husband would be in better spirits. On the contrary, after taking some criticism from Tom about the morality of the procedure, William exploded and yelled at his wife during a celebratory dinner party aboard the Vagabondia.
Afterwards, Jed, who was supposed to be guarding the creature, left his post and attempted to force himself onto Marcia. She was saved when the Gill Man broke free from the operating room and attacked Jed, knocking him unconscious before being recaptured by Tom. Later, the tension-filled expedition returned to California. After the murder of Jed by William for trying to woo his wife and William's own demise at the hands of the enraged Gill Man, Marcia decided that she'd had enough of this life and moved as far away from their house and memories of William as possible, despite developing a mild romantic interest in Tom.