This is a follow-up episode to season 1's Thor's Hammer, where SG-1 destroyed an Asgard device that protected the planet Cimmeria from the goa'uld. Of course, now the goa'uld (a son of Ra and Hathor, called Heru'ur, or Horus the Elder) have invaded the vulnerable planet and are killing and enslaving the population, who believe Thor has forsaken them.
Whil Jack and Teal'c do some recon and fighting, Daniel and Sam follow Gerwyn (the woman who greeted them the first time they visited, and called them for help this time) to see the 'Hall of Thor's Might', in the hope of finding a weapon to fight the goa'uld. They are tested by a hologram of Thor (like the one from Thor's Hammer and solve a riddle showing mathematical prowess, which gives them a direct link to the real Thor, a Roswell-type alien. He has no weapons for them, but returns them to the planet's surface and minutes later sends an Asgard mothership to remove the goa'uld presence from Cimmeria and replaces the Hammer.
This is the first time we really see the extent of the Asgard's power, and the first glimpse of what they're really like. They seem friendly, if a little aloof, but willing to help the fight against the goa'uld - in fact, they've apparently been at war for some time now. They've also been visiting Earth, which is probably what led to the myths of 'Greys', as the similarity is uncanny.
On being returned to the planet's surface, Gerwyn tells the team that the Asgard are going to leave someone with them to help them develop and answer any questions they might have, and that the Hammer will always make an exception for Teal'c. This seems like a blatant invitation for the tau'ri to return and talk to the Asgard, but they apparently never do.
Another thing that annoys me about this episode is that the humans on Cimmeria are at the level northern Europe was at about 1,000 years ago, and this matches with the Norse mythology they believe in, but Thor's test for seeing how advanced they are is seeing if they understand the relation of pi to circles. The value given is '3 14 15 9', and pi was approximated to this level (3.14159) long before the middle ages. Also, pi has no relation to the radius of a circle, which is what Daniel demonstrates. It is actually the relation of a circle's circumference to its diameter.
One more thing we learn in this episode is that Sam, like Cimmerian ex-host Kendra, has retained Jolinar's ability to use goa'uld technology. She sends a blast out from the hand device, but can't get the healing device to work. This is yet another reminder, as in the previous two episodes, that her blending with Jolinar had more effect than anyone realised.