Daniel is doing some research on ancient myths, and believes that there are other aliens who may be friendly. Whereas the Goa'uld were evil and enslaving of humans, the Norse gods were friendly and beneficial towards humanity, potentially a good race of aliens. Teal'c mentions that he has seen mention of Thor's hammer previously, and even knows the address of a forbidden planet, where no Goa'uld is ever to go because of some unmentioned terrible event. The team decides to go there.
As they emerge from the stargate, they find themselves looking at a large pillar with a hammer on top. They also startled some locals, who begin laughing at them and start chanting "Thor, Thor, Thor" as an ominous buzz rises in intensity. Before they can decide what to do, a beam of light shoots out and scans each one of them, before coming to rest on Teal'c. He screams in pain and Jack rushes over to push him out of the way and they both vanish.
Daniel and Carter meet with a local woman who congratulates them on being so smart to get rid of their Goa'uld oppressors by tricking them to come there. They insist that Teal'c was friendly, and want to know what happened to them. The woman thinks this strange, but knows that Thor would not accidentally harm a human to kill a Goa'uld, so they know that Jack and Teal'c are alive.
Jack and Teal'c find themselves in some sort of cave, and as they begin to look around they are greeted by a holographic image of Thor telling them why they have been transported there, Goa'uld technology will not work, and the only way out is through Thor's Hammer, which will only allow the host to leave. They begin exploring the cave and looking for the way out.
Daniel and Carter are brought by the woman to another woman outside of town, who one day was found wandering the mountainside. It turns out she was once a Goa'uld host, but Thor's Hammer freed her from the parasite, and she lives to help the villagers since she can still use the Goa'uld hand device to heal. They convince her (along with the help of ominous thunder, which she attributes to Thor) to lead them to the cave she escaped from so they can go and find their friends.
Inside the cavern, Jack and Teal'c have a run in with the Unas, the first Goa'uld host, who has apparently been living in the cave for centuries. Jack's gun still works, so they blast him and continue on. Teal'c recounts the tale of the Unas and their healing power...but that is only a myth. Turns out, it's not a myth. Jack and Teal'c find the exit, shaped like a hammer. Jack can walk through freely, but Teal'c gets zapped by a red energy until Jack pushes him back out. He cannot escape due to the larva Goa'uld in his stomach. The Unas finds them there, and they figure out that they need to get it into the hammer. Fight ensues, and Teal'c ends up shoving the Unas in.
Meanwhile Daniel and Carter arrive on the other side and hear the gunshots, they get through in time to see the Unas fall dead. Jack decides that they need to destroy the hammer to get Teal'c out, even though it would be useful to free Sha're and Skarra once they find them. Daniel reluctantly shoots the hammer, and a fizzling energy shows that it's deactivated. Teal'c walks out unharmed. But at least they know technology exists that can help them free others from the Goa'uld parasites.
"Thor's Hammer" provides examples of the following tropes:
- Agony Beam: Thor's hammer is this for anyone with a Goa'uld symbiote.
- Alien Blood: The Unas' blood is green.
- Are You Pondering What I'm Pondering?:Teal'c: Are you considering the same tactic as I?O'Neill: Teal'c, the cliché is "Are you thinking what I'm thinking?" And the answer's yes.
- Benevolent Precursors: We get our first glimpse of the Asgard.
- Face Your Fears: The reason Kendra chooses to go back into the labyrinth.
- Fighting from the Inside: Kendra, who due to her training as a priestess, is able to subtly influence the actions of her Goa'uld parasite.
- Foreshadowing: Thor's Hammer disables energy weapons like the Staff Weapon, but does not recognize Jack's gun as being a weapon and leaves it functional. Later installments expand on this.
- Give Me a Sign: The thunder that can be heard intermittently throughout the episode is interpreted by Kendra as a sign from Thor; the persistently convenient timing of the thunder means it's ambiguous as to whether she's correct or not.
- Hope Spot: Thor's hammer is this for Daniel, as it provides the first indication that Sha're and Skaara can be saved. Naturally, it's destroyed by the end of the episode, but he still clings to hope as they now know that it's at least possible for the host to survive.
- Implacable Man: Unas because of his healing powers.
- It Can Think: The Unas looks like a giant beast, but quickly reveals it's both intelligent and malevolent, speaking like Hannibal Lecter as voiced by Darth Vader.
- No Name Given: The dialogue in this episode seems to be deliberately ambiguous as to whether Unas is the creature's actual name, or the name of its species (as well as whether it is literally the first Goa'uld host ever, or simply a member of the first species to be used as hosts). Later episodes involving the Unas would show the latter to be the case, which means the Unas in this episode never gave his name. Supplementary material would later indicate he's named Raux and was Sokar's The Dragon back when Sokar ruled the Goa'uld.
- Sadly Mythtaken: At the beginning of the episode, Daniel refers to Thor’s weapon as "Thor’s Hammer". While Thor’s projection refers to its correct name later in the episode (Mjolnir, for those who aren’t in the know), Daniel treats it as though Mjolnir means "Thor’s Hammer". There are at least three different possible etymologies for Mjolnir, but none of them mean "Thor’s Hammer".
- Sliding Scale of Idealism vs. Cynicism: One of the few times Daniel and Carter actually fight; on the idealistic side, Daniel believes that the existence of Thor's hammer is proof that Sha're and Skaara can be saved and that Kendra can lead them to it; while on the cynical side Carter believes that he is letting his emotions cloud his judgement and that they're wasting time following Kendra when she obviously doesn't know where she's going.
- So Beautiful, It's a Curse: Repeated verbatim by Kendra as beautiful humans are taken to be Goa'uld hosts.
- Special Guest: James Earl Jones voices the Unas.
- True Companions: Teal'c is definitively established as a member of SG-1. Jack refuses to leave him behind, Daniel sacrifices the possibility of saving his wife to free him, Jack refers to Teal'c as family...