The novel's main character, David, is a sixteen-year-old boy who is impulsive and has a habit of over-committing himself. David's chief concern is his infatuation with Angela, "an extremely good-looking and a very enthusiastic type of person,"who is going out with Carl, "a snotty jerk." As the novel opens, David has been thinking about Angela for some time and has finally managed to get a date with her, only to discover that he has already made plans to attend his grandmother's birthday for the same evening. His discovery of the Spee-Dee-Dupe, which makes duplicates of living things, seems to be the answer to his problems, so he decides to make a copy of himself without thinking of the consequences.

During the early chapters of the book, David is mostly concerned with himself and gives little thought to how his actions affect others. The possibility of being in two places at once by using the duplicate is appealing. On several occasions, however, other people realize that unusual things are happening. For instance, David's mother can't believe that he made it home from school in only five minutes.

David is mistrustful of Duplicate A, and he is often confused about his feelings, alternating between jealousy of and sympathy for the duplicate. As the novel continues, however, David is forced to think things through and avoid rash behavior in order to outwit the duplicate. When Duplicate A creates Duplicate B, David must trick them both to keep them from imprisoning him and taking over his life. And in the end, David even begins to feel compassion for Duplicate A and tries to save him from Duplicate B, who wants to kill them both.

Other than David, the characters in the novel are not well developed. They generally remain on the fringes of the action, or in Angela's case, serve as catalysts for David's struggles with the duplicates, who like David are also well developed. At first, Duplicate A appears to be identical to David in every way, although David instinctively mistrusts him. Later, it becomes clear that Duplicate A is not exactly like David—he perceives the duplicate as slightly better looking, more stubborn, and more persuasive. Unlike David, Duplicate A is not afraid of the watch tower, and he also comes to claim control over David. But Duplciate A is also paranoid and creates Duplicate B because he thinks that David is plotting against him. At the end, however, Duplicate A seeks David's help as the cold-hearted Duplicate B threatens to kill them both.

The character of Duplicate B is more handsome than either David or Duplicate A. He is also much more dangerous, being amoral and bent on killing both David and Duplicate A so that he will be the only version of "David" around. Before David learns of his existence, Duplicate B tries to arrange an "accident" by positioning a brick to fall on David's head. In trying to achieve supremacy, Duplicate B pits David and Duplicate A against one another. His attitude is summed up by the words he scrawls on the wall of the tower: "Life is hard. Then you die." In the final chapter of the book, Duplicate B tries to force himself on Angela and invokes Duplicate A's rage. Duplicate B, however, manages to push Duplicate A off the roof of the tower, killing him. But the reader finds that Duplicate B is, contrary to David's beliefs about him, not very clever. When Angela cries out that another duplicate is behind him, Duplicate B turns to face the attacker and also falls to his death.

Although Sleator seems mostly concerned with creating a suspenseful story that will keep the reader's attention, he manages to raise several important issues. At various points, David suggests that his complicated situation has come about because of his impulsive behavior and his mistrustfulness. Because he cannot trust the motives of either duplicate, there is no advantage to having near-clones of himself. More important, however, the novel stresses the uniqueness of all living things and negates the possibility, or desirability, of a duplicate. The novel also plays on the folk theme "Don't wish for something because you might get it." As in the folk tale "The Three Wishes," David's ill-conceived desire for a double nearly brings him disaster. Sleator also makes effective use of the folk character the doppelganger, or evil double, which allows the main character to literally battle himself.