The latest mission assigned to the Enterprise is to rescue a group of scientists from the second planet in the Minaran system before its sun goes into a nova phase and renders the planet uninhabitable. Kirk, Spock and McCoy beam down to the planet but have difficulty in immediately locating the scientific group. The same high level of solar activity that causes their problem also starts to affect systems on the Enterprise and it is forced to leave the area while the landing party continues its search.

An unexpected force transports the three of them into a dark underground vault where they find a young woman lying on a raised platform. Attempts at communication prove to be fruitless as she appears to be mute so McCoy decides to call her 'Gem'. Two aliens appear before them and announce themselves as Vians. When approached by Kirk they knock him out by projecting some kind of force field. The previously inactive Gem goes to Kirk and almost miraculously absorbs the injuries he has suffered.

Spock examines their 'prison' with a tricorder and detects a nearby high concentration of electronic equipment. They start to explore and eventually discover a laboratory where they find the missing scientists who are suspended in glass tubes. Ominously, there are also three other empty tubes. The Vians appear again and demand that one of the landing party take part in their experiments. Kirk volunteers himself and is badly tortured by the aliens who then send him and Gem back to the vault where, once again, she uses her strong empathic powers to absorb his wounds, collapsing with the strain.

The Vians demand another subject for further tests which prompts McCoy to drug the others and let himself be taken by the aliens. Using a stolen Vian device Kirk and Spock locate and follow them but are immobilised with force fields and they have to watch helplessly as Gem struggles to save a dying McCoy. The Vians try to explain the reason for the 'experiments' by telling them that they have the power to save only one of two races that are threatened by the forthcoming nova and are testing Gem to see whether her people are more worthy of being saved. They continue to observe Gem as she grows weaker and weaker, sacrificing herself to save McCoy's life.

Spock manages to break free of his force field restraint and disarms the two aliens. Kirk accuses them of lacking the very qualities which they are expecting Gem to display. The Vians consider this and, deciding that Kirk's accusation has some merit, exercise compassion by using their powers to heal McCoy thereby stopping Gem's deterioration and allowing her to live. They then leave with Gem, having decided that it is her race that is to be saved.