Executive Producer: Gene Roddenberry
Producers: Gene L. Coon + John Meredyth Lucas
Associate Producer: Robert H. Justman
Script Consultant: D. C. Fontana

Writer: Art Wallace
Dir.: Ralph Semensky

Captain Kirk: William Shatner
Science Officer, First Officer, Cmdr. Spock: Leonard Nimoy
Dr. Leonard "Bones" Mc Coy: DeForest Kelly
Lt. Sulu: George Takei
Lieutenant Commander Montgomery Scott: James Doohan
Lieutenant Uhura: Nichelle Nichols
Nurse Christine Chapel: Majel Barrett

Ensign Garrovik: Stephen Brooks
Ensign Rizzo: Jerry Ayres

Stardate: 4307.1

A situation on Argus X reminds Kirk of a haunting experience: He still blames himself for the death of Captain Garrovik, when he was killed by a cloud. Kirk is convinced that, had he fired more quickly, the captain would have survived. His son, Ensign Garrovik, is now serving on the Enterprise and part of the landing party on Argus X, as the same cloud appears and takes many lives again by taking all red blood cells out of the people's bodies. Kirk strongly blames Garrovik, thinking that he had not fired quickly enough and confines him to his rooms. Very clearly Kirk remembers the smell of honey which went along with the cloud and all he had experienced when serving under captain Garraovik on the U .S. S. Farragut. And he keeps on blaming himself. Spock finds out about the captain's reasons to blame himself and speaks with McCoy about it. Now McCoy visits Jim and tries to convince him that the reports on his first job were good calling him a responsible man. Both try to convince Jim that he looses himself in an obsession. 

If you need my opinion I need a drink

Meanwhile the cloud moves on, away from the planet, through space and the Enterprise hunts it. They learn that phasers do not harm the creature. Suddenly the cloud turns round and heads for the Enterprise. It is detected in the ventilation system. Order is given to close all shafts and openings. Again it is Garrovik, who's in trouble. He has thrown something against his opening of the shaft which made it stay open while he thinks it is closed. Spock visits Garrovik and tries to comfort him as they see the steamy appearance of the cloud enter. Quickly Spock pushes Garrovik out of his room and finds out that it is too late to close the opening. 

Outside the room everybody fears Spock is dead by now. Lt. Galloway tells the others that Spock has saved his life. When Spock comes out and tells them that the creature did not like his copper based blood, his friends muse, in great relief, about the creature which would be sick now if it had tasted such blood. 

My blood is based on copper

In Garrowik's room Kirk notices that the creature has changed his smell. He thinks he knows where it is heading for: to his native planet, where Kirk had met it first, 11 years ago. Spock is now convinced, too, that the cloud is a living creature and both convince Dr. McCoy. In a personal moment with Kirk Spock convinces Kirk that having shot earlier would not have killed the creature. He dose not need to blame himself for the death of his crewmates on his first flight. Kirk now talks to Garrowik and assures him that his action, had it been earlier, would not have made any difference. They follow the cloud to it's native planet where it is just about to reproduce itself. For that reason it had been in great need of blood – and still is. 

To use these circumstances Kirk and Garrovik go down and plant a trap. Using a container full of blood just beside a matter – antimatter bomb, they were to be beamed away the moment the creature arrives. Watching the cloud approach quicker than expected, they see the blood gone, while both are still beside the bomb. Thinking Kirk would die when staying there after Kirk sent Galloway back to the ship, he stays, knocks down the captain and wants to stay himself. Kirk only goes down for a moment and assures Garrowik that he never intended to sacrifice himself. He orders Spock to beam them up and shortly afterwards to detonate the bomb. The bomb is detonated and the creature destroyed. Only by using a cross combination of beaming techniques Spock manages to get them back alive because of the explosion at the planets and it's follow up wave of pressure. Now even McCoy is convinced that now and then some logic might be helpful.