#44: The Deadly Years
In case you forgot: The Power Trio gets old. Really old. There's also some drama with the Romulans and THE NEUTRAL ZONE!!! but of course it all gets solved by Kirk.
Sometimes the science in Star Trek is just shaky. Other times it's ridiculous. That is this episode's biggest flaw, and why it isn't rated higher: while a virus that ages you prematurely is shaky (not necessarily outside the realm of possibility; at the least it sounds plausible), the episode's adrenaline cure is total nonsense. Adrenaline doesn't magically shield you from viruses, guys...
Much like the science, Kirk's leadership is by turns inspired and idiotic. Like, has he not heard of a quarantine? The infection shouldn't have spread as quickly as it did if he had followed good medical protocol. But his decision to use old, decoded frequency to bluff the Romulans into backing the hell off is excellent captaining. Would that Kirk was always that on the ball—but then, it'd be rather boring television, wouldn't it?