While investigating a colony along the Romulan neutral zone, a strange disease infects Kirk, Spock, Bones, Scotty, and Lt. Galway (Gee, who's gonna die?), causing them to rapidly age. Now the race is on to find a cure before the aforementioned crew members all grow old and die.
Despite some great makeup effects to make the characters look old, and some standout performances to go along with it, "The Deadly Years" suffers from a total lack of direction and urgency. The episode's main thrust becomes an analysis on Kirk's unwillingness to give up command of his ship when his mental capacity is challenged. But why on Earth, with time to solve the problem so scarce, would Spock allow Commodore Stocker (George Drake) to conduct a competency hearing for Kirk when Spock could simply assume command as an alternative? This competency hearing idea is a sorely misguided attempt at providing "courtroom drama" a la "Court Martial," forcing us to sit through an interminable scene where Kirk is proven incapable of commanding the Enterprise, as characters rehash examples that we witnessed in earlier scenes.
Meanwhile, the race to find a cure is curiously suspended until the hearing is over. This subsequently puts Stocker, the most inept commander of all time, in charge of the Enterprise, which he quickly and ineptly pilots into the middle of a Romulan attack. After Bones comes up with a cure (with an all-too-obvious realization), Kirk saves the day in a nice command sequence. But just how is it curing the disease also reverses the permanent effects of aging?