In search of allies in their battle against the Goa'ulds, O'Neill and the SG-1 team travel to the planet Cimmeria - - home to the legendary Norse gods. But, Cimmeria has long ago been declared off-limits to Goa'ulds and when the team emerges from the Stargate, Teal'c, who as a Jaffa carries an infant Goa'uld within him, is trapped in a mysterious beam of light. When O'Neill tries to save him, both men vanish, transported to a mysterious underground labyrinth. The labyrinth is home to Unas, a vicious creature that is the original Goa'uld host, but it is also Thor's Hammer, which is designed to kill Goa'ulds. As O'Neill and Teal'c fight for survival, Daniel Jackson and Dr. Samantha Carter try to rescue them, aided by Kendra, a former Goa'uld host who survived her own journey through the labyrinth years earlier.
With Kendra's aid, Daniel and Sam eventually find the exit from the labyrinth. Teal's and Jack have made their way there as well, but the "Hammer," a beam of light that kills any Goa'uld symbiote within its host, bars their entrance - Jack can escape but Teal'c will lose his symbiote and die if he passes through. Teal'c throws himself and Unas into the beam - with the death of its symbiote the Unas dies and Jack pulls Teal'c out in the nick of time. Although weapons don't function within the labyrinth, Daniel uses Teal'c staff weapon to blast the Hammer apart from outside, destroying the mechanism. Kendra is leery of what the destruction of the Hammer portends, but the team departs after giving her a box they ask her to give to Thor should he ever return