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It is about a boy named Marco who likes to travel. He often secretly rides buses far from home, though only his telepathic sister finds out. One day, his sister sees strange lights in the basement, and she and Marco investigate. They find a portal into another dimension, and thus the adventure begins. Marco finds strange insect-like creatures there, who are convinced that Marco/Lilly can save their dimension (and as a result save Earth) from their god, which is a naked singularity. But at what cost?
Roger Sutton in his review for Horn Book Magazine said "while the book has the far-out ideas and expert pace that Sleator's fans admire, there's an added dimension of poignancy in the character of Marco, both in his intense bond with his sister Lilly and in his restlessness: obsessed with travel even as a young child, intently riding buses and trains, he has, by the time he reappears as an adult in The Boxes, become something of an intergalactic Flying Dutchman." Diane Roback in her review for Publishers Weekly said that this novel was "more satisfying than its predecessor, and full of strange and startling details, this curious fantasy will spark readers' imaginations and send them right back to The Boxes for a glimpse of Marco's future." Trevelyn E. Jones in his review for School Library Journal said that "Marco is a substantial protagonist; the other characters, while less well-rounded, are convincing enough to advance the story. The book is a successful prequel to Sleator's The Boxes (Dutton, 1998), but also stands on its own." But at this cost Marco will go on this great adventure