As Captain Archer prepares to lead a survey team to an uninhabited planet, Enterprise is suddenly hailed by a trio of aliens who warn him that a deadly neutronic wavefront is quickly approaching. Archer soon surmises that everyone must take shelter in order to survive the storm's radiation. Commander Tucker suggests that the one heavily-shielded place on board that might suffice for the eight-day ordeal is the catwalk, the maintenance shaft that runs the length of each nacelle. Only one problem – the temperature there can reach 300 degrees when the warp coils are online, so he will have to keep the main reactor offline.
With only four hours to prepare, everyone evacuates to the catwalk. The storm soon envelops the ship, and as the days wear on, nerves fray – particularly with the alien guests. To make matters worse, Tucker and Archer discover a problem in Engineering as the injectors have come online. Tucker cannot shut them down from the catwalk, and his EV suit will only protect him for 22 minutes. In Engineering, he soon notices an alien ship docked alongside, and alien intruders, who appear to be interfering with the ship's systems. Doctor Phlox deduces that the aliens are actually immune to the effects of radiation.
The trio confesses that the aliens are actually looking for them. They explain that they were officers in the Takret Militia, but they deserted when they learned that the commanding officers were corrupt. To defend the ship, Archer, Sub-Commander T'Pol, and Lieutenant Reed suit up and while T'Pol and Reed work to shut down the warp reactor, Archer hails the alien leader. He demands they leave, and then orders Ensign Mayweather to head straight for a plasma eddy. As T'Pol and Reed finally succeed in shutting the reactor down, the intruders abandon ship. Soon after, as they clear the storm, the alien visitors apologize for all the trouble they've caused, and depart.