The episode begins with the USS Enterprise entering the Delos system to study a star that is undergoing magnetic field changes. Suddenly, they receive a distress call from another ship. Responding to the call, the Enterprise moves to render aid and finds a battered freighter in a rapidly decaying orbit around a planet. Sporadic communication is made with the ship's captain, T'Jon, who explains that his ship, The Sanction, has lost power. Lt. Yar suggests pulling the ship out of orbit with the tractor beam, but the solar activity causes the beam to fail. Data finds that a coil in the Sanction's engines is damaged and Captain Picard offers T'Jon a replacement. T'Jon is delighted, but is embarrassed to admit that neither he nor his crew know how to install it.
Worf warns that the ship has only minutes before break up, so Picard orders a risky emergency beam out. Yar and Commander Riker proceed to the transporter room and Yar takes over the controls. T'Jon signals they are ready to beam over, but as the transport completes, a cargo barrel materializes on the pad instead of the crew. In disbelief, Yar beams the container to a hold and tries again. With time running out, she locks on to whatever life readings she can find and energizes the beam. In space, the Sanction explodes. Riker then informs the bridge that they only managed to save four of the six crew members. On the pad stand four humanoids; three men and one woman. T'Jon and Romas, appearing scruffy and unclean, are acting nervous and high-strung while Sobi and Langor, appearing well groomed and richly dressed, act completely calm. T'Jon immediately inquires about the container and demands to see it. Not understanding their persistence, Riker leads them to see their "precious cargo".
Upon seeing the container, Sobi and Langor appear gratified; however, T'Jon and Romas are ecstatic and embrace the barrel almost lovingly. Langor casually reminds T'Jon that the barrel is her and Sobi's property, but T'Jon argues, saying they already paid for it with goods in trade. Sobi points out that the goods have gone down with the Sanction and therefore the "deal" is void. The parties verbally bicker until it gets physical, with T'Jon lunging at Sobi and discharging a burst of energy from his hands. Sobi shakes him off and gives T'Jon a shocking grasp of his own. Security arrives and escorts the guests to the observation lounge. Yar admits concern about this strange power, stating that a natural electrical discharge is a difficult "weapon" to confiscate.
In the observation lounge, the calm Brekkans, Sobi and Langor, sit opposite their overtly stressed counterparts, T'Jon and Romas, who are from Ornara. On the bridge, Picard asks Data for information on the two races, but Data states there is little about them on record, indicating that the only known contact was over 200 years ago. A scout ship had reported two inhabited planets with cultures on the verge of space travel; however, neither culture seems to have advanced any further in the past two centuries. Picard meets the aliens and offers his regrets for not saving their ship. T'Jon is curious however, explaining they have two more freighters in need of repair. Data sees no problem in fabricating replacement components if needed. The topic of the cargo comes up and Picard notes that there seems to be some dispute over ownership. T'Jon claims the problem is a matter of life and death, explaining the barrel holds "Felicium", a medicine for a lethal plague on their world.
Langor explains that the plant that yields Felicium grows only in remote areas of his world of Brekka. Sobi adds that it must be painstakingly cultivated and refined, a complex and expensive process, and a single barrel of Felicium represents an enormous investment that they can't just "give" away. Arguments flare again with the Ornarans stating once again that the barrel has already been paid for. Romas says they will soon see what the plague is doing to them when the symptoms take hold of him and T'Jon. Picard becomes alarmed that T'Jon and Romas are carrying a plague, and Romas states that every Ornaran does. Not wanting to risk an outbreak in case the transporter biofilters missed it, Picard initiates a medical emergency and calls for Dr. Crusher.
The four aliens are taken to sickbay, where Beverly runs tests, but from what she can tell, Sobi and Langor are in perfect health. T'Jon and Romas, however, appear to be getting worse, but she can't find the cause. T'Jon pressures Picard, explaining that what is happening to them is happening to thousands on their world. Picard talks to the Brekkans and asks if they'd permit T'Jon and Romas some Felicium for their immediate needs. They kindly offer two doses "free of charge".
Picard takes the aliens to the hold where they are joined by Data and Dr. Crusher. Langor opens part of the barrel and withdraws a circular device which exposes thousands of tiny white pills. She delicately extracts a single pill and places it inside the device for processing. Beverly asks about the dosage, to which Langor answers that it is .01 milliliters. Beverly is impressed with its potency and Sobi explains that advancements in the refinement process have reduced the required dosage, adding that this single shipment would once have filled five rooms the size of the cargo bay. Data asks if technology from their other industries was used to improve the refinement process, but Sobi answers that the Brekkans have no other industry. The Onarans provide them with everything they need in exchange for the Felicium which they produce exclusively â a product which the Brekkans have no use for, but the Ornarans cannot live without. Beverly takes the serum to the sick Ornarans who practically rip the device from her hands. T'Jon prepares a hypo and injects it into his and Romas's hands. Almost immediately, the two aliens begin to relax as if a euphoric bliss has washed over them, making Beverly suspicious. She reports to Picard, stating that she might not fully understand Ornaran physiology, but she recognizes physical reactions to a narcotic and she surmises that the Ornarans are drug addicts.
Elsewhere, Data has uncovered more information about the aliens which explains that the two cultures took different paths; Ornara became technologically advanced and Brekka did not. However, 200 years ago, a devastating plague ravaged Ornara. Somehow, a cure was discovered in a plant indigenous only to Brekka, which resisted cultivation attempts on Ornara. A trading system was established which still exists today; the Brekkans provide the medicine and Ornara provide Brekka with goods and technology (which Brekkans cannot provide for themselves, as their entire economy and industry are dedicated to producing Felicium).
Beverly comes forward with new findings revealing that Felicium had completely wiped out the plague, but the Ornarans have become so addicted to it that they still believe it's needed to survive. Despite the reports, Picard says he can do nothing about it, pointing out that the situation has gone on for a long time and the two civilizations are intertwined in a symbiotic relationship. Beverly only sees it as one race profiting at the expense of another. Regardless of moral sensibilities, Picard states it is not their mission to enforce Federation values on other worlds and refuses to get involved.
Soon a transmission from Ornara comes in; a man named Margan speaks with T'Jon and Romas. Margan insists that T'Jon get the drug to their world immediately, stating that people are dying. The transmission ends and, under duress, T'Jon grabs hold of Riker and stuns him with his electrical power. He demands they be given the Felicium and a shuttlecraft. He threatens that if they refuse, he will kill Riker. Picard tells him he won't be coerced, but T'Jon is very desperate. T'Jon finally relents and lets Riker go. He is near breakdown and tearfully begs Picard to help him.
Sobi and Langor tell Picard they have decided to hand over their shipment to the Ornarans and they can pay them back at some other time as they do not wish to cause further suffering. Picard realizes that the shoe was once on the other foot and asks if the Ornarans ever knew that the plague also occurred on Brekka. The two feign ignorance, but Beverly states the Brekkans realized Felicium caused addiction which remained after the plague was eradicated, but that knowledge was kept from their neighbors. Picard suspects the refinement process has only made the drug more potent so they could tighten their grip on the Ornarans even further. Langor and Sobi become nervous but Picard says the secret is safe since he isn't permitted to tell the Ornarans anything - doing so would violate the Prime Directive. The Brekkans understand that Picard has no choice but to allow the devious practice to continue. Beverly is furious and accuses the Brekkans of enslaving another race just so their pitiful lives would be made easier.
Eventually, the Enterprise arrives at Ornara. Down in the hold, the four aliens prepare to beam down along with their container and the gifts of newly fabricated parts for the Ornaran freighters. At the last second however, Picard says he will not be giving them the parts and states that if their ships are broken, they will have to repair them on their own. T'Jon explains that without the parts their trade cannot continue and Sobi tells Picard that it will disrupt a trade agreement that has lasted for centuries. Langor asks what the Prime Directive says about this, but Picard states that nothing in the directive compels him to offer aid. T'Jon is disgusted, claiming Picard has condemned them to death, but Beverly urges them to find other options. T'Jon states that he hopes Picard knows what he has done to them. Picard replies, of that, he can be sure. He wishes them luck and gives the order to transport the aliens away. Beverly is proud of her Captain, but has sympathy for the Ornarans who will feel intense withdrawal pains. Picard replies that as painful as it will be, the people of Ornara and Brekka must shape their own destinies.