An odd, egg-shaped object, planted by aliens in the mesozoic swamp, is found about 80 million years later by Tycho Tithonis, an eleven-year-old digging in his garden. (Tycho's name, and those of his siblings Ludwig, Leonardo, and Tamara, reflect a deterministic bent on their parents' part that is not irrelevant to the questions raised later.) Immediately, the tyrannical Ludwig demands the object, but Tycho holds on stubbornly, and soon discovers that the "egg" has the power to transport him into the past or the future. At first, Tycho uses the egg to settle small scores with his three siblings, all of whom bully and berate him to an unnatural extent; but he then begins to make forays into the future for glimpses of his adult self. Gradually, Tycho realizes that there are several alternate futures, but in each one the adult Tycho, abusing the power of the egg, becomes more menacing and despicable. In the last visit, he's a crazed "slave" of the egg, controlling and torturing his family. The aghast young Tycho finally realizes that the object itself is behind all the grim developments, and the story ends in a breakneck chase through the eons, with young Tycho, determined to return the egg, pursued by his older self who wants to keep it. Sleator manipulates the family conflicts and their nightmare transformations expertly--building the ideas through the page-turning action, weaving in subtle variations on the dominant theme, and turning to his own nimble advantage those built-in paradoxes of the time-travel concept which often weaken similar stories.