Whilst exploring Cimmeria, Teal'c and Colonel Jack O'Neill are transported to a labyrinth where any Goa'uld will die if they try to escape, leaving the host free. The two nevertheless attempt to find a way out, but are found by an unwanted visitor who will stop at nothing in killing them as it attempts to escape. Meanwhile, Captain Samantha Carter and Dr. Daniel Jackson use Kendra, a former Goa'uld host, to guide them to the labyrinth.
Through examining and comparing Norse mythology to that of the Egyptians, Dr. Daniel Jackson theorizes that there may be another race of aliens who pose as gods, but are good gods, against the Goa'uld, and, thus, a potential ally. Jackson believes that Thor was an alien, and his legendary weapon Thor's Hammer was an alien device that would be an invaluable weapon against the Goa'uld. This possibility is increased when Teal'c makes mention of a world that was banned by the Goa'uld (Cimmeria), which is represented by a hammer sign. The reason why is that any Jaffa or Goa'uld who goes there is never heard from again. Before departing, Major General George S. Hammond gives SG-1 a box to show friendship to the good aliens they will encounter.
SG-1 travels to Cimmeria, and, upon arrival, they encounter a group of people who laugh at them, and then begin chanting the name "Thor" as a giant stone hammer starts to charge.
It scans each member of SG-1 with a beam of light, bypassing Daniel, Sam and Jack before it scans Teal'c with the device, causing him to scream in pain.
When Colonel Jack O'Neill tries to push him out of the way of the beam, both disappear, leaving Jackson and Captain Samantha Carter by themselves.
O'Neill and Teal'c appear inside a cave, where a recording by Thor (in the appearance of a viking) informs them that the Goa'uld in Teal'c's abdominal cavity will die when he tries to leave the cave, revealed to be a labyrinth.
Meanwhile, Jackson and Carter, with help from Gairwyn, who came to see who had come through the Stargate, find a person who had once been in the labyrinth herself. The person, Kendra, was once a Goa'uld host, and so when her Goa'uld had visited the world before, she taunted the symbiote that Cimmeria was full of riches, she was taken to a cave, and, upon leaving, her controller was killed and she was freed.
Inside the Labyrinth, O'Neill and Teal'c realize that they are not alone in the cave. They discover that an Unas, the first host race used by the Goa'uld, is alive in the cave, and he is a dangerous foe. Since Teal'c's staff weapon does not work inside the cave, he and O'Neill rely on bullets to try to kill the alien. Though this appears to kill the creature at first, it is unsuccessful.
As Teal'c and O'Neill reach the exit of the cave, they discover that they were followed by the Unas, and, in order to kill it, Teal'c is forced to push the creature into the door of the cave, killing its symbiote (and nearly killing Teal'c's, too). Carter, Jackson and Kendra arrive soon afterwards, and in order for Teal'c to leave the cave, his staff weapon is taken outside the cave, then used to destroy the Thor's Hammer.
Although, Thor's Hammer could have saved Jackson's wife Sha're and Skaara, Jackson relents to O'Neill's order in order to save Teal'c.
However, this also destroys Cimmeria's only defense against the Goa'uld with Daniel reluctantly firing a shot from Teal'c's staff weapon to destroy the defense, enabling Teal'c who had been trapped to leave the Hall at last.
Before SG-1 leaves the planet, they leave the Sagan Institute Box made of a special alloy for Kendra to give to Thor if he ever returns to the planet, in hopes that they will be able to make contact with him at some point in the future.