This book is ultimately about a quest to save magic. It has all the makings of an epic journey - long travel, many obstacles, singular hero whose payoff in the end isn't as expected. The quest however has two starting points, one for Quentin who, as King of the magical realm, Fillory sets out on a sea voyage to far away islands and one for Julia, a Fillorian Queen whose foray into magic impacts the whole plot. Both characters are actually from Earth, Brooklyn to be exact, lucky enough to be gifted the ability to use magic.
We met both characters in The Magicians as Quentin matriculated at Brakebills, a secret college for magic and then went on to discover Fillory's existence. Julia's history is a bit of a mystery. She learned magic on the "streets" having not gained admittance to Brakebills. We get her complete story here as the narrative jumps from her past to the present which focuses on Quentin.
And that basically covers the general plot. Too much happens to know which details to extract here and take you further into the story. I don't want to give too much away. The basis of the quest is to locate a series of keys. Aside from the final purpose these keys have, they're also able to open invisible doors to other lands. You can go through them but can't return to Fillory since the key always stays behind. In this way, the quest takes Quentin and company back to Earth as well as to the Neitherlands, a sort of transportation terminal with openings between worlds. On these side trips, Quentin reconnects with characters from The Magicians and learns more about the state of magic throughout all worlds.
Grossman turns magic into a gritty, violent, passionate thing. It's not pretty. It's often coated in desperation. This is probably what magic would really be like. I like how the fantasy genre is turned sideways with the fantastical tempered by danger, death, and sadness. You feel like you're getting a complete picture of magic as it would be.
I have no idea where the story will go in the third and final book in this trilogy, but I'm happy to say that I'm interested in finding out.