After detecting an energy spike over a forested area on a distant planet, the Atlantis team decides to investigate. Their travels lead them to a portal built into a rock wall with Ancient carvings and a portal built into it. Intrigued, Sheppard decides to test the portal to see where it leads, and before he can stop himself, is pulled through to the other side with no way to get back or communicate with the team. When no rescue team comes through the portal, Sheppard explores his surroundings and he encounters Avrid (David McNally), a man on the run from a dangerous creature. After a violent showdown, Sheppard succeeds in warding off the creature. Avrid then brings the injured Sheppard back to his home to recover. There, Sheppard discovers that all the inhabitants of the planet are direct descendants of the Ancients, and he is the only one ever to have survived such an attack. Meanwhile, the rest of the Atlantis team is trying to figure out what happened to him. After translating the writing on the rock wiht the help of some Ancient textbooks, Weir (Torri Higginson) discovers that Sheppard has been transported into a time dilation field, designed by the last of the Ancients as a sanctuary from the Wraith -- a place where people could live in peace and saftey and ascend to a higher plane of existence. However, there are also a warning inscribed on the stone: once the threshold across the portal is crossed, there is no return.