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 Captain Archer, Commander Tucker, Ensign Sato, and Sub-Commander T'Pol arrive, they discover a local scientist who believes a local merchant is causing the sickness in the town.
Enterprise locates a planet inhabited by pre-industrial humanoids. Scanners also detect technology which does not correspond with the planet's technological level. Against Sub-Commander T'Pol's recommendation, Captain Archer decides to visit. Going in disguise, T'Pol suggests a distant rural landing site. Once in the Akaali city, Ensign Sato notices inhabitants that appear sick. Scans lead them to an old curio shop, but they encounter a force-field 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 Tucker revisit the shop during the day posing as antique collectors. They confront Garos, asking him why his DNA is not Akaali. After confirming Tucker and Archer are not natives either, he freely admits his origins: he is part of a survey mission from Malur, and the power source is merely a fabrication unit to provide food and clothes. Archer tells him about the illness, but he claims it is an incurable indigenous virus. Doctor Phlox, however, discovers that the water near the shop has been contaminated with a highly toxic chemical, tetracyanate 622.
Archer and Riann observe the shop at night to try and see what is being delivered to Garos. 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. Under the shop, Archer and Riann then discover that Garos is mining a veridium isotope, and the poisoned water is a by-product. Archer orders the reactor beamed up by Enterprise but the Malurians now have a warship in orbit. T'Pol beams the power plant into the alien ship's path, using torpedoes to detonate it and crippling their shields. On the planet, Archer provides Riann with antidote, and assures her that the Vulcans will monitor the planet to ensure the Malurians do not return.