The away teams investigates a strange energy reading when suddenly the puddle jumper's systems collapse. No technology seems to be working anymore. They soon find a civilization of very young people. Their leader Keras tells no one is older than 24 and that on their 25th birthday everybody commits suicide. They believe that will keep the Wraith away, because they aren't interested in them. Rodney has a different idea, he finds the civilization is contained in a field powered by an almost depleted ZPM. He thinks they aren't coming anymore because none of their technology is working inside the field.
While on a first visit to a planet, a puddle jumper crashes when it is affected by an electro-magnetic field. The team finds a colony made up mostly of children and young adults and where the "elders" seem to be in their early 20's. There is a good reason for the current make up of the colony: they believe that it is ritual sacrifice - the shield is unknown to them - that is keeping the Wraith away and as a result everyone voluntarily commits suicide on their 25th birthday. With the colony's current elder on the eve of his own 25th birthday, Sheppard and Rodney McKay have to find a way to convince them that the shield exists and they needn't sacrifice themselves to keep the Wraith away.
Rodney investigates a ZPM from a planet whose inhabitants kill themselves when they turned 25 years old to prevent the Wraiths from finding them profitable.
- Sheppard, Ford, McKay, and Teyla approach a planet but the Puddle Jumper crashes due to electromagnetic interference. Using a compass, McKay homes in on the interference and they find ancient ruins. They are ambushed by teenagers and children who take them to their village. They see a shrine in the village with a Wraith skeleton, then meet the village "elder," a 24-year old Keras. Keras reveals that they made sure the Wraiths would never harvest them by killing themselves at the age of 25.
The team is reluctant to tell the kids that the reason the Wraith no longer come is because of the electromagnetic field. Keras is to die that very night, and his successor, Aries, is concerned about the SG team. While McKay and Ford investigate the ruins, McKay manages to shut down the field and finds a ZPM. Teyla advises against removing it and leaving the locals without protection. They are unaware that the Wraith skeleton that has an active signal device that begins functioning.
Weir is also unhappy with them taking the Z.P.M. and questions if there is a connection between the field and the suicide rituals. McKay determines the Z.P.M. is pretty much useless and the suicide pact is a form of population control because the device couldn't shield the entire planet. Keras asks Sheppard to stand by him at his suicide as they discover the transmitter is functioning. Sheppard destroys it, angering the locals. McKay is unable to get the EMP field working again while Keras gets the SG team released so they can go back to their ship and leave.
They break the truth to Keras while Aries raises up the locals against the SG team as they work against time to reactivate the shield. Aries catches up to them and forces them back to the ship and they go, secretly leaving McKay behind. A Wraith probe shows up and some of the kids double back to spot McKay, and he activates the field just in time. Aries shoots Keras when he intervenes and McKay arrives to tell them they're safe before violence can break out further. The SG team departs on friendly terms.