The Enterprise are sent to check out a conman who is their old friend Harry Mudd. He is trying to sell a fake love potion to miners. Kirk reveals the scheme and ""rescues"" Harry into protective custody. Mudd tricks Nurse Chapel into trying the love potion on Spock as a distraction to take her hostage and escape by shuttlecraft. Spock and Kirk pursue him to an abandoned planet but not before the potion is distributed into the air system. The potion has a delayed effect and Spock soon becomes enarmored of Christine. Complicating matters, a giant rock creature on the planet attacks and the Enterprise crew are too romantically distracted to rescue them. Kirk manages to use the love potion on the Godzilla-sized creature and it spares them long enough to be rescued. The potion's effects totally reverse and the love-smitten duos hate each other for an equal period of time. Realizing he had the real thing on his hands all along and didn't market it correctly, Harry confesses to his misdeeds.