When the Atlantis team loses contact with their allies the Taranans, Weir orders Sheppard, McKay, Teyla and Ronon to investigate. They find the Taranan settlement eerily deserted, but McKay's equipment reveals a single life sign far beneath the ground. The team enters a system of dank tunnels to track it down. Soon they discover a slimy cocoon - empty.
The team resumes their pursuit of the mysterious life sign, but then Ronon notices a side-chamber full of dead Taranans with bite marks on their necks. The team is dismayed to discover an Iratus bug - the Wraith's primitive, predatory ancestor - nearby. Someone has unleashed these vicious bugs against the innocent Taranans.
The life sign draws closer, and the team braces for an assault. In a flash, a horrible creature - half human, half bug - attacks them. It's nearly invincible, and although Ronon wounds it, it escapes down the tunnel. Giving chase, the team discovers another chamber filled with more cocoons, all about to hatch. Sheppard sets off a massive C-4 charge that destroys the creatures seconds before they emerge.
Deciding not to pursue this investigation further without serious reinforcements, the team hikes back to the stargate - but the 'gate won't work. Someone has sabotaged it. Now trapped on the planet, Sheppard leads his team back into the tunnels to solve this mystery.
Underground, Teyla senses a Wraith presence and peels off from the group to investigate. At that instant, the creature attacks again. Ronon shoots the creature point-blank and kills it, but now Teyla is missing. She has been abducted by Michael, the Wraith who has twice been captured by the Atlantis team and forcibly given gene therapy to make him more human.
Now an outcast from the Wraith and furious at humans, Michael has been conducting his own genetic experiments on this planet, mercilessly blending Taranan and Iratus DNA to create a private army of deadly creatures. He is about to unleash an Iratus bug to feed on Teyla when Sheppard, Ronon and McKay arrive. They free Teyla as Michael escapes into the tunnels.
The team must shoulder a heavy burden of responsibility for helping turn Michael into the vengeful monster he is today. As McKay and Teyla study Michael's notes in his lab, Sheppard and Ronon return to the tunnels to track him down. Outnumbered by the creatures of Michael's army, however, they're in for a nasty fight.