As Voyager makes a less-than-successful attempt to vaporize asteroids that are assaulting the Nezu homeworld, a message comes from Dr. Vatm, an astrophysicist who has been analyzing the asteroid fragments on the surface below. Vatm wants to talk to the Nezu ambassador, who is currently aboard Voyager.
Neelix, Tuvok and a Nezu named Sklar head for the surface, but their shuttle crashes. They find Vatm but have no way to contact Voyager. Neelix suggests they reactivate a carriage that is tethered to an orbital space station. If they can rise above Nezu's atmospheric turbulence, they'll be able to communicate with the ship. But as soon as they make the tether system operational, Vatm tries to leave by himself. He's stopped by Tuvok, and the group begins its ascent.
The upward journey is uncomfortable and dangerous. Vatm refuses to say why he attempted to leave without them and sips at water to ease his discomfort. Later, he becomes delirious and says that something is on the roof of the carriage. Then he has a seizure and dies, the result, Tuvok says, of murder; his water was poisoned.
Neelix insists they find out what's on the roof, and Tuvok reluctantly climbs up. He finds a data storage device that contains information about an alien vessel. Sklar pushes Tuvok off the roof, but the Vulcan survives by clinging to an induction coil on the bottom of the carriage. Neelix helps Tuvok back inside and in the ensuing struggle, Sklar plunges to his death.
Finally, the group is able to contact Voyager and is beamed aboard. The crew has been confronted by a ship from the Etanian Order, which wishes to claim Nezu for itself. The Etanians create "natural" disasters — like meteor showers — on the planets they covet, then take over when the residents evacuate. With the tactical information from the device Tuvok found, Janeway is able to disable the Etanian vessel, which departs. After the fact, the crew figures out that Vatm knew there was a traitor among the Nezu, which turned out to be Sklar. However, he hadn't known whom he could trust. His silence ultimately killed him.