I don't think I'm going to give away too much of the plot and certainly not the ending, but if you don't want to know anything more about the plot, stop reading.
Fire is the title character, and she is a young woman living in the Dells. Actually, she's only half human. Her mother was human but her father was a "monster." Monsters in the Dells are the form of normal animals (people, birds, bears, fish, etc.), but they are vibrant colors and are much more dangerous than their regular animal counterparts, and they crave the flesh of humans and other mosters. Fire struggles against her monster nature and deals with her monster powers of being able to control people's minds/wills. She tries to coexist with other humans and live down her father's ignominious reputation. Despite the danger she poses, she's an excellent archer and a great asset, so she ends up being enlisted to help the king's army in their defensive war against two neighboring kingdoms. There's action, romance, and great character development. I don't think it's giving away too much to say that the one character who exists in both Graceling and Fire is Leck (before he became king of Monsea) since he is a baby in the prologue.
I really enjoyed Fire, and I like that each book is related to the other but they stand alone as strongly as they do together. I like that you want to read the next book because you like this one so much, but you are completely happy and satisfied by the new book even though it doesn't continue on with the same store line and characters you fell in love with. Each book is absorbing, satisfying, and complete on its own (yet gives you more of what you like so much!). I will say this is for a slightly older audience. While Graceling was fine for middle school, this would certainly be a bit edgier title for middle grade readers just because there is more discussion to sexual relationships (although the actual sex is all off-screen). Still great for both guys and girls. One of my male students who read Graceling also read my ARC of Fire and said he liked this as much, if not better than, Graceling.