The Enterprise has arrived at the Tellun star system to pick up Ambassador Petri from Troyius, the outermost planet, and deliver him to Elas, the inner planet. The system is on the Federation/Klingon border. McCoy and Spock note that the Elasian men are savage, but the women have the ability to drive men wild with lust. They meet with Petri, who tells them the Dohlman, his people's most deadly enemy, will be beaming aboard. A security team beam aboard and verify the ship is safe for the Dohlman: a beautiful woman. Kirk and the others kneel before her.
The Dohlman is rude and demanding, and Kirk has Spock escort her to her quarters while Kirk speaks with Petri. Petri explains that she is marrying the ruler of Troyius to bring peace to the two people, and his job is to teach her civilized manners. He asks that they proceed at sublight to give him enough time and Kirk agrees. Petri makes sure to point out that Kirk's superiors want the mission to succeed as well and asks him and the crew to respect Elaan.
Elaan rejects Petri's gifts from the Troyius ruler and Kirk goes to respond to her complaint about her quarters, loaned by Uhura. Elaan is not happy that her ruling council is forcing her to go through with the marriage. She orders Petri out but Kirk refuses to cave in to her. Outside, Petri is equally furious but Kirk convinces him to give it another less diplomatic approach.
On the bridge, Spock detects what at first appears to be a sensor ghost but he notes it appears intermittently and isn't an instrument malfunction. Scotty calls up to complain that the Elasians are looking through Engineering and insulting him. Kirk manages to calm her down but is interrupted with a report that the sensor ghost has resolved as a starship. They identify the ship as a Klingon warship that is following them on a parallel course. Kirk then receives a report of a disturbance in Elaan's quarters. He arrives to find that the Dohlman has stabbed Petri in the back.
Petri recovers in Sickbay while Uhura delivers a message that the Federation High Commissioner will be coming for the wedding. Petri explains that Elasian women have tears that cause men to fall wildly in love with them. He insists there will be no wedding and refuses to leave until he has recovered… in several weeks.
Kirk talks with Elaan and tries to get through to her, and takes on the role of teacher for her manners. It doesn't go well.
As the Klingon ship continues to follow the Enterprise and refuses hailing frequencies, Elaan's security chief Kryton is in Engineering. He kills a crewman, Watson, when discovered then sabotages the dilithium cylinders. Kirk goes to Elaan's quarters but when they refuse him entry and draw their weapons, Spock stuns them as the captain anticipated what would happen. Kirk goes in and Elaan refuses to listen at first, but then asks him to help her find a way to get people to like her. She starts to cry and Kirk wipes her tears away, and then falls under her biochemical spell as they kiss.
Uhura detects Kryton sending a transmission to the Klingon ship and they trace it to Engineering. Kirk calls an intruder alert and reluctantly takes his leave of the Dohlman. They've found Watson's body and demand to know what Kryton knows. Kirk calls Spock in to do a mind meld but Kryton grabs a security guard's phaser and disintegrates himself. Kirk orders Scotty to conduct a complete system check.
Kirk informs Elaan of the situation and she notes Kryton probably joined forces with the Klingons out of jealousy over her appointed marriage to another man. Elaan is happy to have the wedding stopped by Klingon interference and she wants Kirk to use the ship to destroy Troyius and make her the rules of both planets. Kirk hesitantly refuses and they're interrupted when McCoy and Spock arrive. They explain about the Elasian women's tears and the fact that they don't wear off. McCoy starts working on an antidote but the Klingons pick that moment to close in. Kirk prepares to order the ship to warp drive but Scotty calls to warn that the anti-matter pods are rigged to explode as soon as they go to warp speed.
The Klingon ship comes at them at warp speed but passes them and they conclude its intent was to lure them into going to warp speed and blowing up. As Elaan comes to the bridge they wonder why the Klingons are so interested in the system. Kirk snaps briefly at Spock before recovering, and then takes her to Sickbay for protection. She tries to manipulate him but he manages to resist her. Scotty says the dilithium crystal assembly is fused and unusable, and they need dilithium crystals to restore warp drive.
McCoy and Chapel are working on a cure while Petri warns there is no antidote. Elaan arrives and Petri convinces her to don the jewels that are her wedding gift, as a sign of respect when they all may die. Kirk returns to the bridge where the Klingons are demanding their survivor. They only have warp drive and limited shields so Kirk stalls for time. The Klingon ship closes in as Elaan returns to the bridge, and the Enterprise is unable to maneuver out of their way. With their shields dropping, Spock detects strange energy readings coming from Elaan's necklace. Spock determines that they are crude dilithium crystals. She explains they are common stones on Elas, and they realize what the Klingons want with the system. Spock takes the crystals to Scotty while the Klingon ship comes in again and fires.
The Klingon ship prepares for another attack run and Spock and Scotty hook up the dilithium crystals. They can't guarantee how the shape of the stones will affect the power flow, but Kirk opens fire and engages the warp drive. At full power, they damage the Klingon and it retreats. They continue on course for Troyius, leaving Elaan puzzled as to why he doesn't finish off the Klingons.
Later, Kirk is in the transporter room as she leaves. Elaan gives him her knife as a personal memento and asks him to remember her. He says he has no choice and watches as she departs. As they prepare to leave orbit, McCoy arrives with an antidote but Spock assures him no cure is necessary: Kirk's bond with the ship has overridden the drug and he's back to normal.