The story begins in medias res with a postcard. Mom, who is a single mother and a paralegal, is preparing for an appearance on the game show $20,000 Pyramid. Miranda, her sixth-grade daughter and latchkey kid, is considering writing a letter to some unnamed "you" who has been sending her letters. This "you" also told her about the game show postcard before it even arrived. Who is this "you" and how is he/she able to predict the future? It's a mystery to us – and to Miranda. Miranda and her mom live in New York City and the year is 1979. Mom has a boyfriend named Richard who is a lawyer and who has one leg that is a little bit shorter than the other – he visits often and helps Mom prepare for her game show appearance. As for Miranda, her favorite book is A Wrinkle in Time, and she carries a copy around with her constantly. There's also a homeless man called "the laughing man," who appears to be a little bit crazy and who lives outside of Miranda's apartment and sleeps with his head under the mailbox.
Moving on… If Miranda were going to write a letter, she tells us, it would all start the day that Sal got punched. Sal is her former best friend who won't walk home from school with her anymore. Miranda narrates the story of the day their friendship ended – the day Sal got randomly punched in the face by a boy in a green army coat. Sal wouldn't talk to Miranda after that and started playing a lot of basketball. From this point on in the book we move between two timelines: Miranda and her mom in the present and Miranda's story of what happened after Sal got punched in the face last fall.
To continue the story of last fall, lots of things start happening after Sal gets punched and he and Miranda stop hanging out. First, Miranda meets the kid in the green army coat on the street. He's kind of weird, can read time without a watch, and is super into math and physics. Miranda's not afraid of him, though. They become something kind of like friends. The two have a conversation in the school's dentist office about A Wrinkle in Time and time travel.
Also around this time Miranda starts to expand her social circle and makes friends with Annemarie, who is on the outs with her former best friend, Julia. Julia is a wealthy privileged girl whom Miranda does not like at all. Annemarie is a budding artist whose dad fusses over her constantly. Miranda and Annemarie start to pal around with Colin, a cute-ish skater boy who gets them a job at a sandwich shop called Jimmy's. Jimmy, the shop owner, is kind of creepy and, we later learn, totally racist. Speaking of things creepy, Miranda gets her first note from "you" that fall when she finds her apartment door unlocked. The note asks where the key to the apartment is weird and that the writer is on his/her way to save Miranda's friend. Miranda finds a second note one day when she's counting a bag of bread rolls at Jimmy's sandwich shop. The note addresses her by her name, says she must write a letter about things that haven't happened yet, and asks her not to tell anyone about the notes. Needless to say, Miranda is freaked out by all of this.
Things start to get complicated with Miranda's new friends. First, we find out that Annemarie has epilepsy and has been hiding it from Colin and Miranda. She isn't supposed to eat anything like bread or soda or the stuff at Jimmy's. She even has a seizure one day in the cafeteria. Julia (Miranda's arch nemesis) tells Miranda about the epilepsy, and eventually Annemarie admits it herself. After Annemarie's seizure, she receives a mysterious rose in front of her door. Who could that be from? Miranda hopes it's not from Colin. Miranda receives a third note that tells her three proofs that she'll be seeing in the future: one is about stolen bread rolls in Colin's backpack, the second is about the inscription on the first-edition of A Wrinkle in Time that she'll receive for Christmas, and the third is about the postcard that begins the novel. All of these, of course, come true.
If this weren't enough, we should also mention that the tension gets ratcheted up with more friend drama. Jimmy ends up making racist comments about Julia and Annemarie flips out and turns on Miranda. This causes a rift. After New Year's, though, Miranda turns the corner and decides to be a nicer person. She reconciles with Annemarie, makes friends with Alice Evans, and calls a truce with Julia. She even goes over to Julia's house to bake a birthday cake for Annemarie. She starts to smooch Colin. She also has a heart-to-heart with Sal about their friendship after he almost gets hit by the truck. A light comes on in her head, and Miranda realizes that the Laughing Man is the one who has been writing the notes to her. Sure enough, in the Laughing Man's discarded shoe is a final note. It says to deliver her letter to him – and she'll know where to find him.
Miranda isn't totally sure what this means until her mother goes on $20,000 Pyramid and wins a bunch of money. Miranda is sitting in the studio audience, and when she sees Dick Clark's ageless face say "Go," her veil is lifted, and she realizes that Marcus and the Laughing Man are the same person – and that's who she should deliver the note to. As the novel closes, Miranda gives her mom applications to law school and writes the letter to the Laughing Man – which she will deliver to Marcus, who is the magic thread.