If someone you loved were taken from you, how far would you go to bring him or her back? This is the impossible question confronting grief-stricken Paul and Jessie Duncan (Greg Kinnear, Rebecca Romijn-Stamos) in the very graveyard where they are to bury their beloved eight year-old son, Adam (Cameron Bright). Into that moment of absolute despair steps Dr. Richard Wells (Robert De Niro) with a calm, reasoned, and utterly incredible offer. He can bring their son back, alive.
He explains that Adam is dead but his cells live on. Wells would clone the boy and Jessie could give birth to him once more, allowing Adam a second chance at life, and the family another chance at happiness. The cells however, will not be viable for long Wells tells Paul and Jessie and the couple has only a day to decide if they can accept this achingly tempting offer. Facing this immediate yet agonizing decision, the couple tries to consider the moral, ethical and legal repercussions of this action. Their love for their son triumphs over all arguments and Paul and Jessie agree to give their boy the chance to live beyond his eighth birthday.
With echoes of a Faustian bargain, Dr. Wells' offer comes with conditions: The process is illegal, so secrecy must be absolute. The new Adam will never see another doctor and the Duncan's will sever ties with friends and family so that no curious eyes will ever see their little boy growing up again. To ensure the secret is kept, the family resettles in the idyllic town of Riverton, close to Dr. Wells' impressive Godsend Fertility Clinic.
At Godsend, Jessie undergoes a relatively simple procedure - just like any woman undergoing in vitro fertilization. The expectant couple is made comfortable with a beautiful, well-appointed, and extremely large home. Paul returns to work with a plum job teaching biology at the local high school. The Duncan's settle in, make friends and eagerly await the birth of their son.
Soon Jessie gives birth at Wells' Godsend Fertility Clinic. The new Adam appears to be a perfect replica in every way, down to very the last cell. His life follows a comfortingly similar pattern until he passes his eighth birthday - and Adam, unaware that he has reached a milestone, literally begins living on borrowed time.
The Duncan's placid life is shattered at first by Adam's screams in the night. The boy, unconscious, is raced to the clinic where Wells diagnoses night terrors, a disturbing but benign sleep disorder that is not uncommon in children. Adam wakes the following morning, apparently fine, but with vague memories of what he describes as weird dreams.
Already gravely concerned for their son, it becomes obvious to Paul and Jessie that something is wrong. His dreams become visions that disturb him as much as his parents. Adam is seized by sporadic mood and personality changes that are shockingly different from the sweet and loving boy they knew through both of his lives.
Paul starts to reconsider his pact with Wells and thinks that maybe it is time to take Adam to an actual pediatrician rather than Wells who, while admittedly brilliant and responsible for Adam's very life, is a geneticist, not a pediatrician. Paul also starts to question Wells' motives, wondering if there could be a deeper or darker truth.
Adam spirals into the world of his visions, seemingly catching glimpses of another place and perhaps even another time. A palpable sense of menace seems to hang about the boy. As he lashes out at home and at school, this beautiful boy begins to seem dangerous. When a schoolmate drowns, Paul forces himself to consider the implication of what they have done and asks himself: how far did Wells really go? Were there darker forces at work as he "played God" with their son?
Paul uncovers the secrets of Dr. Well's past. In a violent and dramatic confrontation, the full, sickening truth comes out.
From this moment on, the Duncan's will have to come to terms with what they have done and what has been done to them.