Village blacksmith Eyck is a big, muscular guy, but he's really a gentle and kind-hearted man who thinks the caste system is outdated. When he sees a low-caste slave being brutally beaten, he has to intervene. He ends up buying the slave and tells the mute teen boy he's free, but the frightened teenager runs away in the night. Five years later, the young man, now grown up and calling himself Wex, show up asking to be Eyck's apprentice. The blacksmith is getting older and doesn't have any children to leave the business to, so he agrees. There's a mutual attraction between the two, but both try to deny it as inconvenient. Wex, in particular, has a hard time trusting people, given the abuse he's suffered.
“The Blacksmith's Apprentice” is an unusual mix of fantasy and science fiction. Most of the story just seems to be a fantasy about creatures that evolved to have very human-like characteristics. But as the plot twists towards its conclusion, there's a greater science fiction element that explains many of the apparent anachronisms.
Wex is a complicated character for fiction. He is mute, and it's not until later that he learns sign language, which he then has to teach Eyck. It creates a big communication problem, which is what leads to some of the dramatic turns of the plot. The young man is also deeply traumatized by his experiences as a slave as well as the years trying to make it on his own after running away from Eyck. His past not only makes it hard for Wex to trust people, but it also keeps him from forming connections. The young man has never had a true friend, someone who wasn't nice to him to get something in return. Being in a relationship seems like being a slave to someone else, and that's something Wex would never allow himself to be.
Eyck is a much more straightforward character. He comes across as an authentically nice guy who always wants to do the right thing. Wex's mercurial nature means Eyck is constantly second-guessing if he's made a misstep with the young man. At times, you have to wonder if the two really can have a lasting relationship.
While this is fundamentally a romance story, there are some thought-provoking elements around racism and classism. The cultural background of the book is a rather rigidly defined caste system based largely on the color of one's fur, which indicates which caste you were born into. There is also a dominant religion that views homosexual relationships as wrong. The story suggests these views evolved but doesn't really explain why.