Now they're about to graduate. Margo comes over to Q's house, dressed as a ninja, and recruits him for a night of wild adventures and revenge. Then she disappears. The rest of the book is about Q's search for her, assisted by his friends and by the clues that Margo has left behind.
Paper Towns has serious themes (do we ever really know anyone? can we ever really know anyone? which cover is the real Margo? or is either of them?), it's very funny. It's also very literary. One of the chief clues is Whitman's Leaves of Grass, which gets plenty of analysis. Moby Dick figures in, too. There's casual sex and casual drinking (tons of it). The ending is maybe not what you'd expect.
What I liked best about Paper Towns was the narrator's voice and the way he thinks about things. Not just Margo but about his last week of high school, the going away, the doing things for the last time. It was long ago for me, but I remember the feelings exactly. The book dragged a bit for me during the long road trip at the end, but that's about my only complaint. I might have to read more of Green's work.