Maj. John Sheppard flies a reconnaissance mission over a new planet, with Teyla, Lt. Aiden Ford and Dr. Rodney McKay with him aboard Puddle Jumper One. The recon is cut short when the craft is brought down by a mysterious electromagnetic field emanating from the planet's surface.
As the downed team searches on foot for the stargate they came through, they discover a ruined city. McKay surmises that the device dispersing the electromagnetic field is in here somewhere. The plan is to find it and deactivate it, so that they can pilot the jumper out of the 'gate and back to Atlantis. But they are ambushed by a group of primitively clothed children and teens armed with bows and arrows. The children refer to Sheppard and company as "full-growns" and take them to see "the elders" at their village - where the team is surprised to see the remnants of a downed Wraith Dart and the skeletal remains of the pilot set up like a shrine.
Then team discovers that these "elders" are all only 24 years old. The oldest by days is Keras, who explains that 500 years ago the Wraith farmed this planet for food; his people were the Wraith's herd; the downed Wraith ship is kept as a reminder. They believe the Wraith feed only on "full-growns," which is why the Wraith have not been back - no one here lives beyond age 24. In addition, they believe that the presence of the Atlantis team, who are all over 24, will bring the Wraith back through the stargate, which they call the "Wraith Well." Keras' 25th birthday is tomorrow - and he is to be sacrificed tonight.
The team is unable to convince the youngsters that it was not their sacrifices that have kept away the Wraith, but rather the city's powerful EM field. At Sheppard's request, Keras lets McKay and Ford, accompanied by the children Cleo and Casta, return to the ruins, where Sheppard says there is a device that will help them fix their ship. Yet Aries, the next elder in line, wants the full-growns gone now and plots behind Keras' back to force Team Atlantis back through the Wraith Well - or, if that fails, to kill them before the Wraith detect their presence.
McKay discovers the EM device, which is powered by, of all things, a precious ZPM (zero-point module). He turns off the device, removes the module, and, after conferring with Sheppard, returns with Ford to Atlantis with the items, hoping to use them to protect Atlantis. Of course, the tribe's protection is now gone, but Sheppard figures they'll be fine for a few hours until the device can be returned.
The homing device bracelet on the Wraith skeleton's hand starts to blink, now that electronic devices are working again. Sheppard smashes it. This is seen as sacrilege. Aries wants them punished, but Keras promises to escort them to the Wraith Well himself.
Back at Atlantis, Dr. Elizabeth Weir tells McKay that taking away the tribe's protection was a bad call. McKay thought they could just whisk the young people to Atlantis, so they could stop killing each other needlessly, and use the ZPM to power Atlantis' shield generator. But Weir suggests that the suicide pact and the field are connected: It stands to reason that since the field isn't powerful enough to protect the entire planet, but only the twelve villages in which the tribes live, that the suicide pact was instilled as population control to keep the people from growing out beyond the field's protective range. Furthermore, the ZPM is only at 50 percent - not strong enough to power Atlantis' shields for more than a few hours, but more than enough to power the city's EM generator indefinitely. Weir orders McKay and Ford to return it and get the EM field back online to protect those people.
Back at the planet, McKay struggles to get the EM device working again, which proves to be much more difficult than switching it off. Sheppard and Teyla show Keras the device and try to explain that this is what has been keeping them safe - not the sacrifices.
Aries shows up with his hunters. Sheppard tells them that McKay is waiting at the puddle jumper, and they were just about to leave. As the hunters escort the team back to the ship, McKay, who was hiding in the ruins, goes back to fixing the device.
A Wraith probe appears. Sheppard and company shoot it down. Aries is convinced that it showed up because the Wraith sensed a "full-grown" crop worth harvesting. Aries' hunters and Sheppard's team are at a standoff, with Sheppard and company exercising extreme restraint not to use their P-90s.
But when a scout sent back to get McKay sees the scientist activate the device, dropping another Wraith probe out of the sky, he runs back and convinces Aries that the device works and that Sheppard was telling the truth. The standoff is over. McKay boosts the power on the device to expand the shield range so that the population can grow again. Keras will live to see his 25th birthday.