Robert Johnson
Felix Locher

Elaine Johnson
Laura Wood

Commodore George Stocker
Charles Drake

Dr. Janet Wallace
Sarah Marshall

Lt. Arlene Galway
Beverly Washburn

Yeoman Doris Atkins
Carolyn Nelson

An hour after speaking with Robert Johnson, the leader of a Federation colony on Gamma Hydra IV, Kirk takes a landing party down for a routine visit and is surprised that no-one is there to meet them. They search around and Chekov is given a shock when he comes across the dead body of an aged man. An old couple approaches the party and identify themselves as Robert Johnson and his wife, Elaine, although they should only be in their late twenties. They are taken aboard the Enterprise but are at a loss to explain their aged condition.

Kirk discusses the situation with Dr. Janet Wallace, an endocrinologist, and Commodore Stocker who is a passenger aboard the Enterprise. McCoy reports that the couple is undergoing rapid ageing but he doesn't know why, and they are all concerned that it may be due to some new Romulan weapon as they are so close to the neutral zone. Despite Stocker's eagerness to continue on to Starbase 10, where a new position awaits him, Kirk decides to stay put until they have cleared up the mystery.

Spock declares the planet to be normal according to the ship's sensors but is trying to gather data on a recent passing comet. Kirk leaves the bridge for Engineering, repeating a routine order that he only issued moments ago. In sickbay one of the landing party, Lieutenant Galway, complains of hearing difficulties. She is joined shortly by Kirk who has experienced a second instance of forgetfulness and an unusual twinge in his muscles. An increasingly greying and irritable McCoy diagnoses mild arthritis and they are still pondering the oddity when a wrinkled, white-haired Scotty arrives.

The entire landing party seems to be affected by the ageing condition, although it is less noticeable in Spock due to the Vulcan's longer life span. Chekov seems to be the only member of the group who remains unaffected. Eventually, Spock discovers the cause of the condition to be due to radiation in the planet's atmosphere left by the passing comet. Commodore Stocker becomes concerned about the captain's ability to command and orders Spock to take charge and relieve him of duties. Spock refuses as he is also suffering from the same condition, causing Stocker to convene a competency hearing.

In the meantime Lt. Galway, who has aged more rapidly than the others, dies of old age in sickbay. After hearing evidence, witnessing Kirk's failing memory and assessing the condition of the other command officers, Stocker assumes command of the Enterprise even though he has no experience of starship command, ordering helm to set a course for Starbase 10 which is directly across the neutral zone. Relieved of duty, Kirk joins Scotty and McCoy in sickbay where they are working alongside Spock, trying to find a remedy for their condition.

Kirk recalls that Chekov was shocked and scared at his discovery of the body on the planet which prompts McCoy to remember that adrenaline used to be administered to counteract radiation poisoning. They immediately start work on formulating a treatment. On entering the neutral zone the Enterprise comes under attack from a Romulan patrol vessel. Stocker doesn't know how to respond and his attempts at communicating with the Romulans are ignored as they continue their attack. Knowing the situation, and with no time to waste, Kirk administers an untested dose of serum on himself that they have formulated, screaming in agony as it takes effect.

On the bridge, panic is setting in as the ship's shields begin to fail. The Romulans are gathering themselves for a final attack run when a rejuvenated Kirk walks onto the bridge and assumes command from a relieved Stocker. He orders Uhura to send a message to Starfleet Command using a code that he knows the Romulans have broken. The message advises them that he intends to set off a corbomite device in one minute which will have a devastating, far-reaching effect on that area of space and that all Federation ships should avoid the area. The Romulans, as hoped, crack the message and rapidly back off, leaving the Enterprise free to make a run for it out of the neutral zone at warp eight. By the time the Romulans realise that they have been conned it is too late for them to intercept the Enterprise again.