After a long night of work on an Ancient device found in Glastonbury, England, Carter achieves a breakthrough. As Mitchell looks on, she activates the device - which unexpectedly shifts them both into a different dimension. Invisible and powerless, they can now only observe as their friends try to figure out where they've gone. A distress call from the Sodan warriors interrupts Teal'c and Daniel's search. Teal'c and SG-12 hurry to the Sodan homeworld, where a brutal massacre has taken place. The Sodan leader, Haikon, clings to life as he explains what happened. A Prior of the Ori turned one of the warriors, Volnek, into an irrational killer. Wearing a Sodan wrist device that makes him invisible, Volnek butchered his comrades and is still stalking the area. Because warriors wearing the wrist devices can see each other, Teal'c puts on a second device and sets out in pursuit. On Earth, Dr. Lee discovers lepton radiation near the Ancient device. Daniel, recalling that leptons were associated with his own disappearance into another dimension several years ago, delves into Ancient texts recovered in Glastonbury. There he finds a legend about Arthur's Mantle, a mythical cloak that made the famous king invisible. The cloak, Daniel concludes, was actually a medieval metaphor for the Ancient device, which allows transportation - and communication - between alternate dimensions.As Carter and Daniel begin a painstaking, trans-dimensional conversation, Mitchell tags along with a team of reinforcements to the Sodan world. There, Teal'c has just confronted Volnek in a desperate battle, but Volnek was eerily invincible to gunfire and forced Teal'c to retreat. Meanwhile, Dr. Lee activates the Ancient device - mistakenly sending Daniel to the alternate dimension, too. Frustrated, Daniel nonetheless seizes the chance to read pages of holographic text that the device only displays on this side. He learns that Merlin, an Ascended Ancient, built the dimensional transporter to keep a critical project secret even from his fellow Ancients. He was designing a weapon to defend his people from the Ori -- a weapon that can kill Ascended beings.