One of the stand-alones, with only a few allusions to other works (and a few future works with allusions to it) in Discworld. Also, I think, one of the weaker ones. "See, I can depict gods as dorks" is somewhat weaker as a gag than some writers realize.
But we open with an eagle dropping a turtle in a garden. The turtle is in reality the god Om, trapped by lack of genuine belief. Fortunately for him, he lands near the novice Brutha, his last believer. Meanwhile, the Deacon Vorbis, in charge of the Quisition, is preparing a holy war. He discovers Brutha has a perfect memory and sets out to use him.
It involves philosophers, who are the sorts of people who jump out their baths and run about without a stitch when they get bright ideas, digging for water, miracles, deals, a steam-powered ship, a labyrinth, discussions of whether the Turtle moves (the joke on their believing the world is round is also a bit thin), and much more.