Ruth Thomas was the fiancee of Edward Parker. When her betrothed did not arrive in port, she enlisted the help of the fearless Captain Donahue to go looking for him. After interrogating Captain Davies and learning what had happened to Edward, they found their way to Dr. Moreau's island.
Leaving Donahue's ship anchored offshore the pair went inland where they were greeted cordially by Moreau. Despite a friendly dinner it quickly became apparent that Edward was the scientist's prisoner and so were they all. Worse, one of Moreau's creations, Ouran, took a keen interest in Ruth and attempted to force himself on her, as per Moreau's scheme to mate a human with one of his beast people. She was rescued by Edward and Donahue however.
Subsequently the three decided to escape. An attempt by Donahue to reach his ship and return with his crew resulted in his death, seemingly dooming Ruth and Edward. However the killing, ordered by Moreau himself, contradicted his own "Law," prompting the beast people to rebel. Guided by Moreau's disillusioned assistant Montgomery, Ruth and Edward were able to slip away during the revolution and escape to safety aboard Donahue's ship.