Marcelo Sandoval has a pretty good life, at least at the beginning of the book. He may have some kind of Asperger's-type syndrome, but he's happy. He goes to Paterson, a school for special-needs kids, and he straight up loves it. He's just gotten a job working with Paterson's equine-therapy horses, he lives in a nice house (well, actually, he lives in a fancy tree house in the backyard), and, best of all, he hears his own imaginary internal music (or IM) whenever he wants. In short, life's pretty sweet.
Enter Arturo, Marcelo's dad. He's been there all along, of course, but now he's about to throw a giant tire iron in the spokes of Marcelo's life. You know that job at the stables? Yeah, Arturo's not down. He thinks that what Marcelo needs is a dose of the "real world," which amounts to one of two options: either he goes to Oak Ridge High instead of Paterson in the fall, or he gives up his job at the stables for the summer and comes to work at Arturo's law firm, Sandoval and Holmes. If Marcelo does well at the firm—and "well" is defined by Arturo, of course—he can go back to Paterson for his senior year.
Marcelo chooses the law firm, though he's less than pumped. He'll be working in the mailroom with Jasmine, the manager. Jasmine's got it together: she's only two years older than Marcelo, but she's running the place. She's not happy about the situation either because she wanted to hire a girl named Belinda, but Arturo forced Marcelo on her.
What really makes the law firm awful, however, is Wendell Holmes, the son of Arturo's partner Stephen. Wendell's just graduated high school and is going to Harvard in the fall. He's already become the captain of the squash team, he has a yacht, and he's pretty much a pompous, womanizing jerk, just like his dad.
He wants to hook up with Jasmine, who can't stand him, because she has a brain. So Wendell, scheming cad that he is, seizes the opportunity to manipulate Marcelo. He'll pretend to be Marcelo's friend and will tell Arturo good things about his son, which will mean Marcelo can go back to Paterson in the fall and get out of his way. All he wants in return is for Marcelo to a) do his work for him, and b) lure Jasmine onto the yacht so he can put the moves on her. (Marcelo eventually wises up and refuses, which, whew.)
Marcelo's been at the law firm just long enough to become friends with Jasmine, who turns out to be a very smart, music-loving, kindred spirit, when Wendell goes to Arturo and drips some sleaze on him. And by "sleaze," we mean "charm," but the sleaziest possible kind. He wants Marcelo to work with him (read: for him) on the Vidromek case. Vidromek is Sandoval and Holmes's biggest client, so Arturo's totally jazzed. Marcelo wants nothing to do with it, but of course Arturo thinks this is majorly real-world and will hear none of his protests.
Marcelo grudgingly gets to work organizing the Vidromek files. He's doing as he was told, tossing duplicates in the trash, when he realizes that one of the things he's thrown away is a picture of a severely deformed girl. Her face has been ripped halfway off by shattered glass. Oh, and did we mention that Vidromek makes windshields that were supposed to shatter into harmless pieces on impact? Yeah, not so much.
Our hero suddenly comprehends suffering in a way he never has before. Sure, he's known disabled kids at Paterson and dying kids at the hospital where his mother, Aurora, works as an oncology nurse. He's also obsessively studied world religions, most notably with his mentor Rabbi Heschel (even though Marcelo's Catholic). But he's never seen anything quite like this. He knows Sandoval and Holmes have wronged this girl in the name of protecting Vidromek, and he knows he has to help her get justice.
He and Jasmine put on their Sherlock Holmes hats and do a little digging. Long story short, the girl's name is Ixtel Jaetz, and her lawsuit against Vidromek is being handled by Geronimo "Jerry" Garcia, another Mexican-American attorney who was Arturo's classmate at Harvard Law. It's the classic good guy vs. bad guy tale: Garcia remained true to his ideals of helping people, while Arturo became corrupted by greed and went the big-money route. Garcia wrote to Arturo and asked him to settle out of court so Ixtel could get reconstructive surgery, but no dice: Arturo wrote a mean letter back and basically told Garcia to shove it.
Despite knowing that doing the right thing will condemn him to public high school and could financially ruin his father, Marcelo finds a file saying that Vidromek knew the windshields were defective, and he turns the info over to Garcia. Arturo has no choice but to settle out of court. When Marcelo finally meets Ixtel, she thanks him profusely for helping her, and he knows he's made the right decision.
It's not exactly a happy ending, but one thing makes it happier: he and Jasmine are soulmates, and they decide to move to Vermont together. The book ends with the sweetest kiss ever—Jasmine plants one on Marcelo's cheek—and the IM, which Marcelo had lost when he entered the real world, floods over him once again.