Wallace "Lolly" Rachpaul lives with his mother in St. Nicholas Housing Projects in Harlem, New York City. His best friend Vega lives in an apartment nearby and is someone Lolly can always count on especially in supporting Lolly after the sudden and tragic death of his older brother, Jermaine, who was shot in the Bronx on Halloween. Lolly's parents are divorced and his father has not been around much. Knowing that Lolly has been struggling with his grief, Lolly's mother's girlfriend Yvonne gifts him two big garbage bags full of Legos for Christmas.
Lolly attends an after school program every day with Vega and does his homework amongst his friends from the Saint Nick projects. Lolly also talks with the director Mr. Ali, who is a social worker that helps Lolly process his grief over the loss of his brother. Mr. Ali also offers Lolly the opportunity to build his Lego creations in an empty storage room. Lolly looks forward to escaping to the storage room every day and has soon built an elaborate make believe world that he calls Harmonee. Lolly invents complex stories and even interactive games to go along with his world. One day Mr. Ali allows Lolly's classmate Rose, who struggles socially due to her autism and is also grieving a loss, to come to the storage room and build alongside Lolly using his Legos. At first Lolly balks at the idea and resents "big Rose" invading his sanctuary. Lolly even challenges Rose to a ten feet tall tower building contest to see who is better at building with Legos. The champion would get to stay. However, the contest ends in a tie and Lolly and Rose are forced to continue working together.
During this time, Lolly and Rose begin to come to an understanding and start communicating through their love of architecture. Rose has an uncanny ability to reconstruct buildings she has seen exactly. While Lolly's world is fantasy, Rose's is realistic. Her intricate constructions are replicas of the Saint Nick Projects and the surrounding areas. Rose is also grieving the loss of her mother to suicide. She is raised by her grandmother who has told her: "Rosamund, when you die, they bury you, but your soul flies to the stars. Your mama, your daddy—they were buried under the ground, but they’re stars now, girl, stars beneath our feet.” Rose takes this to heart and has placed stars on the bottom of her buildings to represent all those who’ve died.
Rose and Lolly’s creations begin to attract a lot of positive attention. Their friends and families are amazed at what they’ve created. One day Mr. Ali tells them that the city has mandated the storage room be used as an exercise facility. Unfortunately Lolly and Rose have to tear down their Lego worlds. Lolly is devastated. Creating Harmonee has been a coping mechanism and the thought of destroying it makes Lolly feel like he is losing Jermaine all over again. Before the Legos are torn down the after school program arranges for them to have a display for the community. Many people come out to see the worlds that Rose and Lolly have created. Lolly even makes it onto the news! He is able to see that he might have a future one day as an artist!
One day while Lolly and Vega are out getting food at a bodega for Vega's mom, they get jumped by two gang members who have been harassing them. Lolly‘s new phone and Vega's new jacket are stolen and both boys are badly beaten up. The attack leaves both boys hurting and angry. While Lolly has his Legos to escape to, for the first time Vega feels disillusioned and his mind clouds over with thoughts of revenge. Vega even procures a gun. When Lolly sees what Vega is planning, he tries to talk him out of it. Lolly also wonders if it would be easier to join a gang for protection. He is tired of feeling afraid every day and feeling like a victim. However, both boys decide that they don't want to go down the same path as Jermaine. Vega and Lolly go to the river where Jermaine was shot and throw the gun in giving up on revenge and choosing the non violent path.
The story ends with Lolly realizing that even though he will always miss his brother, he can find positive ways to channel his grief. He can keep his brothers memory alive by talking to him in his mind and finding others like Mr. Ali to talk to when he needs it. He realizes that there are a lot of positive influences around him that he can lean on. Lolly thinks, "The folks you hang out with can raise you up or bring you down low. Over time, they can make you think a certain way - change who you really are.” Lolly decides to work hard to choose a different future than what Jermaine suffered. He realizes, "I had learned the most important thing: the decisions you make can become your life. Your choices are you.”