Called upon to lend their technological expertise to a village on the planet Taranis, the Atlantis team finds a civilization powered by geothermal energy. Seemingly unconcerned by strong tremors that have been rocking the planet, the village leader, Lycus, seeks help correcting a problem with their shield generator. Sheppard is more than happy to assist, especially after Lycus reveals that they are in possession of a massive, yet disabled, Aurora Class warship. However, as McKay begins work on the generator, he finds that the village is headed for disaster, and that the tremors are linked to an active super-volcano that could erupt at any moment.

At Weir's insistence, Lycus sets aside his suspicions and agrees to evacuate everyone to Atlantis until they can safely relocate to another planet. But when the weakened ground gives way during the evacuation and the gate is destroyed by a lava flow, Sheppard and company lose all contact with the outside world and are left stranded dangerously close to the volcano. Sent in to help, Colonel Caldwell sets about using the Daedalus to begin the evacuation. But when it?s clear that there's not enough time to ferry everyone safely to Atlantis, their only hope lies in reactivating the Ancient warship. As McKay gets to work, things again take another turn for the worse when a lava flow traps them beneath the surface of the volatile planet.

As McKay focuses on repairing the aging warship, Teyla, Ronon, and Beckett discover that, despite the pressing danger, not everyone on Taranis is willing to leave. As delays leave them stranded by the planet?s toxic atmosphere and unable to return to the evacuation site, two large fissures that have opened up near the base now threaten to make it impossible for the ship to take off. With both time and oxygen in short supply, Caldwell manages to beam Teyla, Ronon, Beckett and their refugees onto the Daedalus. And just when it appears as if there is no way out for Sheppard and the rest of the evacuees, McKay devises a brilliant, albeit risky, solution that uses the mega-volcano?s eruption to launch the warship into orbit where they are saved. However, back on Atlantis, Weir realizes that the approach of a Wraith hive ship could mean that they are about to face yet another battle for survival.

When Atlantis is contacted by an approaching hive ship, they are shocked to discover that it is carrying none other than Michael, the Wraith that they attempted to turn into a human. Though initially wary of his intentions, interest is sparked when Michael claims that the Wraith are being divided by a civil war and says that they want to use Beckett's retrovirus to turn their rivals into humans to feed upon. And in return, Michael and his Wraith Queen promise to turn over valuable technical information about their weapons and defenses.

Soon after the team accepts the terms of the deal and agrees to provide assistance, Michael is joined on Atlantis by the Wraith Queen and her chief scientist. Once Beckett reveals that he has found a way to transform the retrovirus into an aerosol gas, the Wraith insist that he help devise a plan to disperse it inside their enemy?s ships. And holding up their end of the bargain, they give McKay a treasure trove of information about Wraith technology that he needs to complete the job.

Yet, as is soon discovered, this unusual partnership is too good to be true. While Ronon still harbors deep suspicions about Michael, others on the Atlantis team hope that the alliance will buy some time and help protect humans on other worlds from attack. Yet, once the assault on the enemy ship is underway, Weir and her team discover that they have played right into a trap. Following the crash of Zelenka?s computer terminal and an attack on the Daedalus by both Wraith ships, it becomes clear that the alliance was just an elaborate ruse. Finally, with McKay and Ronon taken hostage aboard the hive ship, Zelenka uncovers a worm-like virus that was uploaded into the Atlantis computers, giving the Wraith access to information that will make possible a devastating attack against Earth.

cadwell has been taken control by a Goa'uld on one epsiode.