An away-team discovers a station of researches afflicted by a condition that rapidly ages them to death by natural causes. The exploration of "old age" in the external conflict of this episode acts as a spiritual precursor to the internal conflicts portrayed later in the Star Trek films.
On the base level of technical execution, the limited makeup capabilities of the time are paired with varyingly convincing performances from the main cast. Along with this, an inappropriately placed romance subplot may elicits giggles. Thankfully, restrictions of time or budget do not doom this episode to the fate of the worst.
The concept is loaded with the inherent terror of psychological deterioration. However, the broader thematic implications of the internal conflict of coming to terms the aging process are left relatively untouched. While these would be deeply explored in the cinematic realm with the Star Trek films, here it is strictly a dramatic conflict of the plot's novel conceit.
While the experience of lesser premises might suffer from lack of deeper theme exploration, the structural execution here is thorough and sufficiently effective. In characterization, however, little of what has thus far defined these characters in the series seems to survive their afflictions, creating a sort of dramatic barrier that makes them difficult to accept at the crew we love; Kirk in particular.
This distinct lack of connection with their un-radiated selves places the affliction more in the realm of possession or madness when performed. While still dynamic, this choice of tone seems unbefitting of the problem's core attributes.
The only glimpse of Kirk as we know him comes when he elects to take the first dose of an experimental remedy that could kill if not heal. It's a terrific, almost reverse-werewolf, devolution back to his younger self, who storms the bridge just in time to assume command during a Romulan attack!
It's a classically Kirk moment, showing up the top brass of Starfleet while single-handedly saving the day. It's a tight if not shallow bit of adventure plotting with a fantastical challenge-of-the-week, which falls just short of true Trek greatness. "The Deadly Years" has aged relatively well.