The number 11 should be one of Sam MacKenzie's favorites; after all, it's the date of his birthday. But for some reason, Sam is afraid of it. On the night before his 11th birthday, he climbs out of the house he shares with his grandfather, Mack, and shimmies up to the attic in search of his birthday presents. Instead, Sam finds an old metal box with a piece of a newspaper hanging out of it. A picture in the clipping is of Sam, when he was about 3 years old. Unfortunately, not only is most of the story hidden inside the box, but Sam is dyslexic, unable to read more than a few rudimentary words. He does make out that the headline claims Sam was missing, and that his last name used to be Bell.
Sam's imagination begins to run wild: What if his grandfather isn't really his grandfather? What if he was kidnapped? He doesn't fully believe it, but he sets his mind to finding someone who will help him solve the mystery. He decides to ask Caroline, a new girl in school, for her help. He gets his chance when their teacher pairs them together for a school project to build a medieval castle. Sam proposes that they build the model out of wood, using the tools in his grandfather's woodworking shop. Caroline agrees.
Sam celebrates his birthday with his grandfather. Their friends Onji and Anima, who own the deli and the Indian restaurant in the same building as his grandfather's woodworking shop, also help him celebrate. Onji and Anima are like family to Sam. Onji makes Sam's lunch for school every day, and Anima brings them dinner every night. Anima gives him a book about woodworking that uses more pictures than words for directions. Onji gives Sam an oversized T-shirt advertising his own deli. But Sam's grandfather gives him the best present of all: a plane for smoothing wood, which has been passed down in the family. The celebration ends with Anima telling a story, as she does every night, but this time, Sam's grandfather seems upset at her choice. She tells an old Iroquois legend about the creation of islands floating in a great river. Sam has the feeling he has heard the story before and that he remembers the river, too.
Sam tells Caroline about the newspaper clipping, and she agrees to help him find answers. She also encourages him to think hard about his past. His mind can probably remember more than he believes it can, if he'll just concentrate. A few days later, when Mack's grandfather is away on business, Sam and Caroline sneak up to the attack. They manage to unlock the box, and Caroline reads the clipping. It says that Sam disappeared in a boating accident. As she reads the story, memories begin to resurface in Sam's mind about struggling in water.
Later that night, Sam climbs back up to the attic and opens the trunk. This time he finds a toy sailboat. Its masts remind him of the number 11. Sam knows he played with the boat when he was younger. He also finds another piece of paper. After studying it for some time, he figures out that it's a schedule for a ferry, and he is determined to find out where it came from.
Sam brings the papers to Caroline at school. She finds the address of a children's home on 11th Street on one of them. Sam has more memories that seem to suggest he once lived somewhere else with other people. He and Caroline believe he probably lived at the children's home. He dreams of a castle and loud footsteps. When he wakes up, he decides to use the castle in his dreams as a model for the castle he and Caroline are supposed to build.
One night, when Mack is out working, Onji has Sam over for dinner. He tells Sam about how he and Mack used to go fishing all the time when they were younger. Sam's curiosity is sparked because his grandfather has always said he hates the water. Onji rambles for a moment more, exclaiming how Mack used to love to fish and swim and sail but then, as if remembering something, he stops talking. Sam tries to question him further, but Onji manages to change the subject at every turn. Sam remembers a type of fish Onji said he and Mack used to catch. He looks it up on Onji's computer several days later. When Onji's daughter sees what he's looking at, she tells Sam that the men used to catch that fish up in The Thousand Islands in the St. Lawrence River. Sam realizes that this area is not that far from where they now live.
The next day, Caroline tells him that her family will be moving away the following weekend. Sam is devastated. They won't get to finish their castle or solve the mystery surrounding his life. In his grandfather's shop that afternoon, a customer admires the castle and remarks that it looks like a model of the castle on Heart Island, near The Thousand Islands. Caroline calls to say that her mother will let her spend the whole Saturday with Sam before they move. She has a plan to use her baby-sitting money to buy bus tickets up to The Thousand Islands and find the children's home where Sam used to live. Unfortunately, when they arrive, the home is abandoned, but Sam remembers more about the time he spent there.
After saying a tearful goodbye to Caroline, Sam goes to Anima to ask her about his past. She tells him about the night he came to live with them. Onji had come in a few days earlier to tell her that his best friend, Mack, was moving up from Florida and needed a place to live and work. He would be bringing a child with him as well. Onji and Anima prepared the house for them, but Mack didn't arrive when they expected. They waited all night, but Mack didn't come until dawn. He and Sam had been soaked through. Mack cried and cried and wouldn't tell them what happened. Eventually, Anima was able to pry Sam from his arms and dry him off. They never spoke about that night again but accepted Mack and Sam into their lives so that they were like family.
When Mack sees the finished castle, he is dumbfounded. It is a perfect replica of Boldt Castle, a landmark that Mack had worked on when Sam was younger. Mack takes Sam up to The Thousand Islands and tells him about his past. Mack's daughter (Sam's mother) used to take Sam up to visit his grandfather as he worked on the castle. But one day, Mack and his daughter argued. Mack can't recall about what, but he became so angry, he quit his job and moved away. When Sam's mother died, the authorities weren't able to locate Mack, so they put Sam in the children's home.
Later, when Mack was found and his daughter's will delivered to him, giving him custody of Sam, Mack hurried back to get his grandson. The woman running the home told Mack to come back in the morning. She was tired and didn't want to get Sam ready to go. Mack was furious; he stomped through the house until he found his grandson, and then took him out in a raging winter storm. Mack had planned on sailing his boat down to Onji's house, but it crashed on some rocks and overturned. Mack managed to save Sam, and they took a train to Onji's. Mack realized it was his anger that had caused him to lose his daughter and almost lose his grandson, as well. Since that night, he vowed not to let his anger get the better of him again, and he never had. He and Onji alerted the authorities that Sam had survived the accident, but Mack kept the newspaper clipping because he planned on one day telling Sam all that had happened. Sam is relieved to know the truth, especially that Mack really is his grandfather.
Sam brings the model castle into school for his class's medieval feast. The teacher and students are impressed with Sam's work. He explains how Caroline kept the journal of their work and made up the knights and princesses that inhabited the model. He is overjoyed when Caroline arrives to enjoy the feast. Her mother drove her down for the occasion. Before Caroline leaves again, she begs Sam to try to learn to read and write so that he can email her. He returns to the resource room and asks a teacher to help him. She agrees to work with him after school and throughout the summer to improve his abilities. That night, Sam writes his first email to Caroline, just one word, "yes."