Kirk attempts to secure a mining agreement for topeline on Capella 4. Coincidentally, McCoy had been stationed on the planet for two months, and warns Kirk that although the Capellans are scrupulously honest, they are a war-like people, 7 feet tall and extremely strong. Their conventional weapon is a throwing-star type weapon called a kleeat. Before beaming down with McCoy, Spock, and a red-shirted security officer, Kirk warns Scott that the Klingons are thought to be active in the Capella 4 sector.
Upon beaming down, this prediction is born out. As McCoy greets the Capellans, the landing party learns that the Klingons have arrived first and are presumably already engaged in negotiations. Before he can be restrained, Brad the security guard pulls out a phaser to shoot the Klingon emissary and is instantly killed by a kleeat-wielding Capellan. After the landing party agrees to hand over its weapons and devices as the Klingon have already done, they are treated as honored guests. A Capellan woman offers Kirk some food, but Kirk is luckily restrained by McCoy before touching her, since it turns out that touching a Capellan woman mandates hand-to-hand combat with her closest male relative. In fact, Kirk's refusal to initiate combat greatly disappoints the woman's closest male relative.
Kirk and the Klingon negotiate with Tierr Akaar, leader of the ten tribes of Capella. While the Tierr seems to favor the Federation, Maab and the Tierr's consort favor the Klingons. The Tierr is then challenged to a duel by Maab. Before this can take place, a factional fight for power take place. In the fighting, Akaar is killed and Maab becomes Tierr. Upon becoming Tierr, however, Maab's perspective changes, especially after seeing fear in the Klingon's eyes when Kirk challenges him to a fight.
Meanwhile, a Klingon vessel diverts the Enterprise with a phony distress call from the freighter S.S. Diadra (maximum speed: Warp 2). To Scott's surprise, upon arriving at the scene, he can find no trace of the distressed Diadra. However, he notes that the distress call asked for the Enterprise by name, despite the fact a freighter would not have knowledge of its whereabouts. As the Enterprise speeds back to Capella, the Klingons try another diversion with a distress call from the U.S.S. Carolina. Scott ignores it, citing the saying ``fool me once, shame on you, fool me twice, shame on me'' (which Chekov reports is an old Russian saying). However, before the Enterprise can return to Capella, it is confronted by a Klingon warship.
On Capella, Akaar's pregnant wife Eleen is sentenced to death because she carries a royal child. Kirk intervenes, in the process antagonizing the Capellans and Eleen herself, who demands his death for laying hands upon her. The Enterprise landing party is put under guard with Eleen, and McCoy prepares to treat her wounded arm. The landing party takes the opportunity to overpower their guards and run away to save Eleen. They hide out in the hills, where McCoy treats Eleen's arm, and also discovers that she may give birth at any time. Eleen begins to become interested in McCoy.
Using their communicators, Kirk and Spock set up an avalanche which blocks and kills some of the Capellans tracking them. The Klingon uses the opportunity to steal one of the confiscated phasers from a wounded Capellan, and then stabs him to death. Kirk then finds a cave in which to shelter Eleen, but the climb there is difficult. Eleen will only let McCoy help her make the climb until McCoy remarks ``I'm a doctor, not an escalator.'' In the cave, they make a fire using a magnesite nitron tablet. While McCoy delivers the baby, Kirk and Spock construct bows and arrows.
After giving birth, Eleen hits McCoy on the head with a rock and runs to give herself up to the Capellans. She claims to have killed the child and the Earth men as they slept. The Klingon does not believe her, and demands of the Capellans under threat of phaser fire that they verify Eleen's story. Suddenly, Kirk shoots the Klingon with an arrow, and an exchange between the Capellans and Kirk and Spock follows. The Klingon threatens to shoot anyone who raises a weapon against him. This does him no good, however, since Maab exchanges his life for that of Eleen by confronting the Klingon, and Kieel uses the opportunity to kill the Klingon with a kleeat.
After this fracas has taken place, Scott and a landing party arrive. When McCoy re-unites Eleen with her child, he confounds Spock with his use of ``obscure earth dialect'' oochie-woochie-coochie-coo. Kirk gains mining rights when the Eleen acts as regent for the child Tierr-to-be, named Leonard James Akaar.