The Enterprise investigates a pre-industrial civilization of about 500 million people. They discover that there is another warp-capable species among the unsuspecting inhabitants. After Archer, Trip, Hoshi, and T'Pol arrive, they discover a local scientist who believes a local merchant is causing the sickness in the town.
The Enterprise finds a planet inhabited by 500 million pre-industrial humanoids. Enterprise scanners also detect a possible anti-matter reactor on the surface, which does not fit with the rest of the planet's technological level. Against T'Pol's recommendation, Captain Archer decides to visit. They go in disguise, but T'Pol still has misgivings and suggests a distant rural landing site, to minimise the risk of cultural contamination. Once in the Akaali city, Hoshi notices a couple of Akaali that appear sick. Trip and Archer's scans lead them to an old curio shop but they encounter a forcefield blocking the way. They are then confronted by a local apothecary, Riann. T'Pol stuns her and when she awakens, Archer persuades her that he is an investigator from another city.
Archer and Trip revisit the shop during the day posing as antique collectors. They are greeted by the owner Garos. Once the shop clears of other customers they confront Garos asking him why his DNA is not Akaali. After confirming Tucker and Archer are not natives either, Garos freely admits his origins. He is from the Malurian system and was part of a survey mission to study the Akaali but he decided to stay. He claims the power source is merely a fabrication unit to provide food and clothes. Archer tells Garos about the illness and Garos claims it is an incurable indigenous virus. Phlox, however, discovers that the water near the shop has been contaminated with the highly toxic chemical tetracyanate 622.
Archer and Riann observe the shop at night to try and see what is being delivered to the Malurian. In the course of their conversation, Archer's translator malfunctions and he is unable to understand Riann's native tongue. He resets his translator while suddenly kissing her to hide his device from view. Soon they follow a man leaving the shop with a delivery. He leaves the crates in a forest clearing, and Archer is then attacked by a Malurian, and is forced to use his phase pistol. Riann witnesses the events from a distance and realises that neither are Akaali.
Archer and Riann then gain access to the back of Garos' shop and discover that Garos is mining a veridium isotope. The poisoned water is a by-product of this mining. The captain orders the reactor beamed up by Enterprise but the Malurians now have a warship in orbit. T'Pol reacts by beaming the power plant into the alien ship's path and using torpedoes to detonate it, crippling their shields. On the planet, Archer is engaged in a phaser fight with three Malurians, finally forcing them to beam up to their ship. Before leaving, Archer gives Riann medicine to cure the afflicted Akaali. He also assures her that the Vulcans will monitor the planet to ensure the Malurians do not return. The two then share a goodbye kiss.