The creatures (who communicate telepathically) tell Marco he must go on a giant swing and retrieve a small bag at the very top.

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.

"[1] 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.

"[2] 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.

In an email to a fan in 2006, he wrote the following: "Some day I would like to write a third book in that series, called MAD MARTHA.