After Simon Van Gelder's escape from the Tantalus Penal Colony, McCoy assigned her to investigate Van Gelder's injury. She and Captain Kirk visited the colony to talk with Dr. Tristan Adams and learn about the accident.
She was able to advise Kirk about the neural neutralizer. While not familiar with the specific equipment in use on Tantalus, she was familiar with the concept, and assured Kirk that Adams had not created a "chamber of horrors" in the neural neutralizer – a statement that was later revealed to be horribly wrong.
After hours, Kirk enlisted her aid in a hands-on demonstration of the neural neutralizer. Convinced she would be able to evaluate the effects and determine whether the device was injuring him, he acted as the subject, while she operated the equipment. They had discovered the device was far more effective than initially suggested when Adams and Eli overpowered her and began to employ the device to mentally condition Kirk. Included in this conditioning were powerful romantic feelings towards Noel, built on their chance encounter at the Christmas party.
When Kirk awoke from his treatment he fought off his feelings long enough to dispatch Noel through the air conditioning ducts to the power station. There, she managed to shut down the power to the colony, enabling Kirk to escape the treatment room, and Spock to beam down with a team and secure the colony. (TOS: "Dagger of the Mind")