Thor’s Chariot is episode six of season two of Stargate SG-1.
In the first season episode “Thor’s Hammer” the SGC had their first encounter with the Asgard, another, friendly alien race, who also posed as gods, although they only witnessed a recording, showing Thor himself. Although the image isn’t exactly true to life to say the least. They had to destroy Thor’s Hammer, which protected the residents of the planet of Cimmeria from the Goa’uld in order to free Teal’c without killing him.
After it looks like the Goa’uld have attacked the planet, SG-1 return to the planet to investigate and find it has indeed come under attack and there is a Goa’uld occupation force. Sam’s brief stint as a host of a Tok’ra in “In the Line of Duty” has given her the ability to use Goa’uld technology.
In this episode Thor reveals his true shape – he’s a Grey. Why are the Asgard Greys? Greys don’t have the most sterling of reputations. Maybe that’s explained in a later episode I missed the first time.