Two locals are making their way through the wood at night when they see someone up ahead. Approaching the figure, it turns and the two men run away in a panic. One gets away only for Michael, wearing a gas mask, to attack him.
A year later, an Atlantis team led by Major Anne Teldy finds the gas mask in an underground facility. They discover something else and call Atlantis. On the base, Carson is preparing to leave for Earth while McKay tries to convince him to stay. They're interrupted when Sheppard comes in to inform them the team has reported the discovery of one of Michael's labs. Sheppard and Beckett meet Teldy's team on the planet and go into the laboratory, where they see the stasis pods. Dr. Alison Porter has disabled the security systems and goes to work with Beckett to open the stasis pods, with Sgt. Mehra standing guard. Sheppard, Teldy, and Vega go to check out the local village.
Beckett discovers the stasis pods hold more of Michael's hybrids, which mixed in several other life forms. Sheppard and the others find the nearby village seemingly deserted. They're unaware that something is watching them. Meanwhile, Beckett determines the stasis pod subjects were considered a success and they're no longer people.
Sheppard and the others find one villager, Mirellus, who says that people started disappearing a year ago and the inhabitants left, believing the place was cursed. Mirellus is from a nearby village and followed them to give a warning if necessary. Beckett contacts them to suggest they bed down at that village and exchange notes. Mirellus prepares to leave, saying it's not safe to be there at night.
Mehra escorts Beckett and Porter through the woods toward the village and hears something. She dismisses it as nothing, while something follows behind them. That night, Sheppard, Teldy, and Vega leave for the neighboring village while in the lab, one of the stasis pods activates on its own and something emerges.
Sheppard's group is walking through the fog when their lights start to go out. A figure moves through the mist around them. Back at the village, Mehra leaves Porter and Beckett to bond while she goes outside and checks the perimeter. She finds a well and drops a rock in, then finds a child's doll with a hole in its face. As she walks away form the well, mist starts to rise from it.
Sheppard and the others find Mirellus, who warns about the demon in the mist and says they're all in danger. Vega is suddenly snatched away and they go after her, only to find that she's dead. He spots the figure again and when it comes out of the mist, he shoots it down. More figures close in and Sheppard orders a retreat. Beckett, Porter, and Mehra hear the gunshots and Mehra orders them to stay in the building while she investigates.
Sheppard, Teldy, and Mirellus retreat to the laboratory where they discover the pods have been opened. Mirellus explains he opened the pods: his wife was among the missing and he though she was in on of the pods. Unable to raise the others on the radio, they go to get Porter. At the village, Beckett and Porter hear something outside but Mehra is nowhere to be seen. Beckett grabs a gun and goes out to investigate, and a figure approaches him. It notices him and he runs off, tripping on a branch. The figure approaches him, emitting mist from it gills, but doesn't seem to see him until his radio goes off. It attacks him but Mehra arrives and shoots it down then gets him back to the village building. Porter is gone and Sheppard and Teldy arrive. Beckett concludes the creatures are emitting the mist which may have long-term toxic effects, and that Michael couldn't control the creatures. They split up and go outside to search for Porter, and one of the creatures emerges from the well. Sheppard finds Porter and heads back to the building. Mirellus and Beckett are arguing in the building when the creatures grab Mirellus and pull him out the window. The mist starts pouring in under the door as the creatures try to break in. Outside, Mehra finds Sheppard and Porter, and they spot Teldy's glow stick. It turns out to be one of the creatures with the glow stick thrust into its shoulder. Mehra guns it down and they get back to the building and find Teldy.
Beckett is running through the mist, while Teldy concludes that a tunnel runs from the lab to the town through the well. There are nine of the creatures left and Porter suggests they use her computer to make an ultrasonic noise so they can lure them to the well and then blow it up. They move into position with Sheppard putting the computer in a bucket. He activates the detonator for the C-4 on a five-minute countdown… and one of the creatures attack him. Sheppard manages to shoot it and Beckett arrives, drawn to the gunshot. They search for the explosives without success, and then retreat as the creatures move in, attracted by the sound. The explosives go off and Porter, Teldy, and Vega open fire, killing the creatures. They move in to search for Sheppard and Beckett, who contacts them via radio and reveals they went down the well and are in the catacombs beneath.
Reunited, the team checks out the stasis pods. While Sheppard, Teldy, and Beckett prepare to destroy the info, Mehra and Porter hear something moving in the darkness. Sheppard finds a hidden compartment and he and Teldy go inside and find more opened stasis pods: more of the creatures are out. They then hear gunfire as one of the creatures attacks Porter and Mehra. Sheppard tells Teldy and Beckett to head for the gate as more of the creatures charge at them out of the mist. They gun them down and Porter and Mehra emerge from the other end.
Back on Atlantis, Beckett is packing again when Sheppard comes in to see if he needs help. Porter is recovering and Beckett suggests she be assigned to work with him. Sheppard agrees to pass it on to Woolsey while McKay wants to know who Beckett's new friend is.