When Klingons threaten the Enterprise, Captain Kirk needs his best weapons officer on the bridge, but Lt. Chekov is incapacitated with a debilitating disease that is causing him to age rapidly... a disease for which Dr. McCoy can find no cure. In this story by veteran Star Trek writer D. C. Fontana, actor Walter Koenig returns as Pavel Chekov, reprising the role he made famous. Guest starring Mary Linda Rapelye, from the Original Series episode "The Way to Eden." Written by D. C. Fontana. (thetvdb.com)