Young veterinarian Rachel Goddard's world begins to crumble when a client rushes into the animal hospital with a dog struck by a car during a thunderstorm. The woman's terrified child, drenched with rain, loses sight of her mother in the flurry of activity and screams, ''Mommy! I want Mommy!'' Instantly Rachel is hurled back in time to a day in her own childhood when her baby sister, Michelle, uttered the same cry while thunder crashed and rain poured down on them. The unearthed memory feels like a fragment of a nightmare, and Rachel doesn't understand its meaning or the anguish it stirs up in her.