Margaret, a sculptor, has been hired by a museum to build a life-sized model of "Lucy," mankind's ancestral link to the primate world. Such an opportunity couldn't have come at a more fortuitous time.

Margaret's life is in disarray: Her strained, loveless marriage has just ended, her finances are in a mess, and, to top it all, the dentist announces that she needs complicated and expensive dental work. Her sense that she is withering away spiritually and emotionally, and now the horrifying feeling that her physical self is also decaying, plunge her deeper and deeper into sleep and despair.

The meticulous work of reconstructing Lucy - a process resembling a dissection in reverse - draws Margaret away from herself and then back again. Bone by bone, muscle by muscle, Margaret must cross boundaries of time and space to create an exact replica of Australopithecus afarensis. She is guided in her work by several sets of footprints found fossilized in volcanic ash.

Those belonging to a male hominid walk straight ahead, but the prints of the female - Lucy - turn off the path for an instant, telegraphing across three million years a moment of hesitation. What, asks Margaret, was Lucy thinking and feeling? Fear? Curiosity? Longing? As Margaret casts herself back in time to ask the question "Who are you?" she finds herself identifying more and more with her human ancestor.

And in doing so, Margaret is forced to explore fundamental questions about evolution, the human condition, and her own troubled and perplexing life.