Man, trying to sum up 'Eragon' without spoilers is a trip. So it's basically a farm boy who finds a mysterious blue stone that hatches into a dragon, Saphira. He gets drawn into a rebellion against the evil king Galbatorix after his uncle is killed by these creepy supernatural soldiers called the Ra'zac. Classic hero's journey stuff, but it really leans into the bond between dragon and rider as its heart.
Key twists? The big one is Brom, the old storyteller who mentors him, is actually a former Dragon Rider himself, which explains all his knowledge. That reveal lands perfectly. Also, the villain Durza, a Shade, isn't the final boss at all; you realize Galbatorix is this looming, distant threat who corrupted the whole order. The twist with Murtagh at the end of the first book is brutal—finding out he's the son of another traitorous Rider, Morzan, and then having him get taken by the enemy. It sets up a much more complicated conflict than just good vs. evil.
I always come back to how the magic system, based on a magical language, gets tied into everything. Eragon's first major spell, when he tries to bless a baby, goes horribly wrong because he didn't understand the wording. That moment showed the rules had real stakes.