The planet Gideon has finally agreed to allow one Federation representative to visit and negotiate a treaty. Captain James Kirk of the U.S.S. Enterprise has been assigned as the Federation Delegate and beams down from the starship to co-ordinates supplied by the Gideons to attend the talks. On materialising, he is very confused to find that he is aboard what appears to be a completely deserted Enterprise with no memory of what has occurred for the last nine minutes and with a strange bruising on his arm.

The Gideon ambassador, Hodin, contacts the Enterprise and asks Spock why Kirk has not beamed down, which causes some concern on the bridge as no faults have been reported by transporter control. Spock decides to beam down and search for Kirk but Hodin vetoes this, forcing Spock to contact Starfleet in order to gain official permission. Meanwhile, Kirk roams the deserted ship and encounters a beautiful young woman called Odona wandering about, seemingly mesmerised by the size of the ship and the amount of space.

On hearing an unusual sound he opens a viewing port to see many faces watching him. Odona collapses and Kirk takes her to sickbay where he meets Hodin who is apparently quite pleased to see that she has been taken ill. He explains to Kirk that there is no infection or sickness on Gideon and that, together with their beliefs preventing them from practising birth control has led to disastrous over-population.

Kirk was purposely chosen by their council to bring sickness to the planet as his blood contains elements of the deadly Vegan choriomeningitis which he had survived. The starship that they are apparently on is a huge mock-up built on the planet's surface. Aboard the Enterprise Starfleet have refused to give Spock authority to beam down to the planet he decides to do so anyway, using the same co-ordinates that were given by the Gideons for Kirk's transport.

He finds the three of them in sickbay on the false starship and they have Odona beamed up to the real thing aboard the Enterprise where her illness is treated. She now has the infection in her blood as well and goes back to her planet carrying the disease that she believes will give her race some hope for survival.