The oldest known space faring race, the Vedala, summon six alien specialists to their asteroid. Two of them are Spock and Kirk, who arrive via the Enterprise. They beam down and meet with the Vedala representative, who introduces them to the other four specialists: Tchar, hereditary prince of the avian Skorr; Sord, a super-strong reptoid; Em/3/Green, a cowardly lockpick and thief; and Lara, a huntress with a perfect direction sense. Spock has been chosen for his scientific expertise, and Kirk for his leadership abilities.
The Vedala representative has Tchar explain that his people were warriors who became civilized two hundred years ago thanks to the influence of Alar, a religious leader and teacher. Upon his death, Alar's brain patterns were placed in an indurite sculpture, the Soul of the Skorr. Now someone has stolen the sculpture. The Skorr, believing only an alien could have committed such sacrilege, are preparing to make war upon the entire galaxy. Spock notes that, given Skorr breeding patterns, they could raise an army of 200 billion warriors in just two years. The Skorr government has hushed up the theft but they must find the Soul before the Skorr people go on a holy jihad.
The Vedala have traced the sculpture to a "mad planet": a Class M planet wracked by cataclysms and temperature changes. The identity of the thief is unknown, and three previous expeditions to the plant have been wiped out. The Vedala can't survive there and need others to take on the mission. Kirk and the others agree and the Vedala teleports them instantly to the planet. The Vedala provide a transport vehicle with necessary equipment. Tchar can't sense the soul but the vehicle has a sensor tuned to indurite. Em/3/Green gets a sensor lock but then the readings fluctuate and the panel explodes. However, Lara gets a fix on the coordinates and Spock concedes her abilities must be perfect or the Vedala wouldn't have chosen her.
Spock checks their supplies and Tchar flies ahead, while Lara flirts with Kirk. Spock determines that the Vedala provided them with weapons and Kirk wonders why when the Vedala didn't mention any inhabitants. Spock notes that they are there, and there may be other guardians. The team proceeds in their vehicle toward the indicated coordinates as the weather abruptly changes to blazing sunlight. Tchar spots a fortress up ahead and sense the Soul within it.
Volcanoes erupt and lava advances upon their position faster than the vehicle can move. Kirk suggests they bypass the power pack safeties for one high-speed run and Em/3/Green volunteers to do the rewiring. Working with Spock, the insectoid alien works as lightning speed to make the adjustments. Meanwhile, Tchar flies overhead and finds a spot to slow the lava flow. Kirk and Sord create a landslide to block a ravine and they return to the vehicle. They proceed at speed but when they hit a rock Spock is thrown out of the vehicle. Kirk goes back to rescue him and they manage to get back to the vehicle and outrun the lava flow. The engine burns out and they're forced to proceed on foot.
The weather turns icy cold and Em/3/Green almost falls into a crevasse. The team rescues him but the alien pleads that he's too tired and cold to continue. Sord carries him against his wishes and Tchar scouts ahead as they proceed through a forest. Sord spots something but Spock notes there are no life forms and Kirk assumes Sord is imagining things. They spot the fortress and Kirk insists on scouting it out. Lara insists on going with him and flirts some more, while Spock notes that the Vedala would have told them if there any life forms on the planet. Sord isn't so assured of their host's omniscience.
Kirk and Laura find the fortress and the others catch up to them. The fortress is based on primitive Skorr temples and Tchar finds the entrance. Em/3/Green examines the door and determines that it is locked and booby trapped. As he works to open the entrance before the timed explosive goes off, large flying creatures attack from above. Tchar struggles with one of them in the air while the others open fire. The phaser fire causes the creatures to blow up, revealing that they're robots. The remaining creature carries Tchar over the top of the fortress. With no way to ascend the smooth sides of the fortress, they have no other option but to go through the door once Em/3/Green finishes his work.
The inside of the fortress is barren except for a narrow ledge spiraling up the wall to the Soul of the Skorr, suspended in a force field high above the floor. The door seals behind them with no way to open it. Kirk isn't surprised, figuring that someone must have destroyed the three previous mission teams from within. Everyone but Sord ascends to the ledge and begins edging their way up to the Soul. They get close and Lara suggests lassoing the Soul, but it would simply bounce off the force field. Suddenly a phaser beam fires, destroying the ledge behind them. Kirk realizes only one individual could have placed the Soul so far above the floor: Tchar.
Tchar flies down, boasting that he stole the Soul knowing it would drive his people to jihad. Contemptuous of the peaceful ways of Alar, Tchar wants to see his people regain their proud warrior traditions, no matter the cost in blood. Tchar grants the team the right to die as warriors and activates a gravity neutralizer. They all drift in the air so they can fight as Skorr does. Kirk realizes that if they violate Tchar's rules and use their weapons, Skorr will use the fortress phaser banks against them. He and Spock, skilled in zero-g maneuvering, launch an attack at Tchar and manage to restrain the surprised Skorr. Kirk manages to hook his foot on the Soul of the Skorr and Lara activates retrieval for the Vedala to teleport them back.
Back on the Vedala asteroid, the Vedala representative informs them that the imprisoned Tchar will be cured of his madness. The mission, and the temporary theft of the Soul, can never be mentioned or acknowledged. Sord wonders if there'll be any questions but the Vedala representative assures them that there will be no questions and in time they'll forget what happened. Lara has just enough time to express her regrets about that to Kirk before they beam back to their respective ships. Once aboard, Kirk and Spock discover that only a few minutes passed for everyone else. Kirk says that the Vedala changed their minds and the Enterprise departs.