In 2273 aboard the freighter Skorpius, they put their plan in motion. With the aid of a medical droid, Zauber used a forming pod to alter his physical appearance and voice into that of McCoy. Zauber then polluted the water supply of a fuel depot to trigger an outbreak of Rigellian fever. Joann intentionally infected herself with the deadly disease. A distress call from the depot lured the USS Enterprise. When McCoy boarded the freighter to treat its crew, she paralyzed McCoy. Zauber took McCoy's clothes, locked him in an isolator unit, and assumed his identity. He had Joann and the isolator unit beamed up to the ship.
Once onboard, he told James T. Kirk he would retire when they reached the next starbase, and told Spock he wanted his share of patent royalties for himself. His lack of empathy, particularly for ailing Zaranite passenger Zantra, prompted Christine Chapel to study his perscan records, which showed him to be a completely different person. Spock proved with a mind meld that the real McCoy was locked in the isolator.
Meanwhile, within a sealed surgical room treated with a fluorine-enriched atmosphere, Zauber prepared to perform open heart surgery on Zantra. Kirk, Spock and McCoy confronted Zauber, who threatened Chapel's life with a laser scalpel. Zantra awoke and yanked out Zauber's oxygen supply. He breathed fluorine gas fumes and collapsed. McCoy said Joann and Anton both would need psychiatric help. (TOS comic: "The Real McCoy")