The movie opens on Ian (voiced by Tom Holland), an elf turning sixteen in a world of modern technology a lot like ours - you know, cars, cellphones, but also centaurs and pixies. And we learn that magic exists, but has been forgotten in favour of technology. The movie lays it on thick that his mother (Julia Louis-Dreyfus) and older brother Barley (Chris Pratt) embarrass him horribly (although they love him and he loves them). His father died around Ian's birth. And then we see him at school, where they again overload us with the knowledge that he's socially awkward, has trouble standing up for himself, and doesn't believe in himself. This is about 20 minutes of the movie, spent humiliating the main character to show "he's a nice guy but a wimp."
Then, for his sixteenth birthday, his Mom gives him and his brother a magician's staff his father wanted them to have - along with a spell that will resurrect their father for one day. And we soon find that Barley's role-playing game obsession has left him with a knowledge of magic - but no skill, whereas Ian has the skill but no knowledge. In casting the spell, they end up with the lower half of their father, and go on a quest to try to finish the spell.
The movie is about family, and faith in family (and to some extent about sacrifice, again for family). The director's commentary (with director Dan Scanlon and his producer Kori Rae, who had less to say) on the DVD made it clear that he built a lot of the movie on the experiences of himself and his brother after their father died very young. Some of the movie's strongest moments revolve around the relationship of the two brothers - when they're not deliberately making the older brother a dick.
I really disliked Scanlon's previous movie, "Monster's University:" I think it's one of Pixar's poorest movies. Sure, "Cars 2" holds down the bottom end of the Pixar scale, but "Monster's University" tried to reach those depths. Happily, this is a (mostly) charming and family-friendly movie, although lacking Pixar's usual zingers-for-the-adults jokes. It's a likeable movie, but pretty weak and disappointing on the Pixar scale.