Adam Gibson (Arnold Schwarzenegger) wakes on his birthday with his lovely wife. Adam's daughter informs him she wants a Simdoll, a lifelike, life-sized doll that acts almost human. The parents notice that the elderly family dog is not well, and express concern about it dying and how sad their daughter would be. It can be replaced by Repet, an animal cloning service, but Adam doesn't trust clones. Cloning knowledge is held by the multinational corporation Replacement Technologies.
Human cloning is technically possible, but illegal. Adam thinks his daughter should just learn to accept death as a part of life, rather than being cushioned from it. Arriving at his helicopter piloting business, he and his partner Hank Morgan (Michael Rapaport) learn they are to transport Replacement Technologies CEO Michael Drucker (Tony Goldwyn). Oddly, the CEO's staff requires the pilots to sign several forms, as well as a small blood test and an eye screening, before the flight can be undertaken.
Bemused, the pilots submit to the tests. Adam's wife calls with the news that the dog was so ill it had to be put down, and begs him to get a Repet for their daughter. Adam will not commit to it. Drucker has specifically asked for Adam to fly him due to his reputation, but ambitious Hank begs to be the big man this one time.
Adam agrees, introduces Hank to Drucker as Adam Gibson, and Hank flies Drucker. Unfortunately, their landing atop a snowy mountain is met by a group of fundamentalists who believe that cloning is immoral and Drucker must die. The party is wiped out. Adam awakens confused in a taxicab that he does not remember entering. He assumes he fell asleep, and directs the cab to the Repet center so he can find out about cloning the dog.
After hearing the sales pitch, he becomes convinced and is about to order the dog when the salesman shocks him by saying he remembered Adam coming into the shop earlier. Adam is disoriented and decides to get the Simdoll for his daughter instead. He returns home and is about to open his front door when he sees through the window - a surprise birthday party, starring a man who looks just like him, with his family and friends. Before he has chance to find out what is happening, he is grabbed by a group of thugs. These people plainly want him dead. After a drawn-out chase by car and on foot, he loses them and goes to Hank's condo. After Adam encounters Hank's virtual girlfriend and pet cat Sadie, Hank arrives.
However, they are interrupted by another religious fanatic who accuses them of being abominations. The two men return to Adam's home to see what is happening. While Adam is busy elsewhere, Hank watches Adam's wife in the detached garage giving the Adam-lookalike his birthday present, cigars (which are illegal and an incredible crime to possess), and then seducing him in their car. Adam is livid. He realizes the other Adam must be a clone, but he does not want to disrupt his family's peace by charging in to have it out with the guy.
In the meantime, the thugs continue trying to kill him. When they die during attempts, Drucker clones them and puts them on the mission again. All is not well within Replacement Technologies. Drucker is working on a US Senator whose young son is dying, offering him a human clone of the boy for manipulative purposes. Dr. Griffin Weir (Robert Duvall), the inventor of cloning, is unhappily discovering the seamier side of the business. His poor wife is dying from a horrible illness..
. and evidently not for the first time. She begs him not to clone her again, saying that she has suffered enough. He discovers that Drucker has been deliberately inserting defective gene sequences into clones so that they "expire" within five years, in order to keep "control" of the illegal human clones. Weir wants out - Drucker refuses. Exasperated by the failed attempts to kill Adam, Drucker has his wife and daughter picked up and brought to Replacement Technologies headquarters. Adam is forced to ally with his clone to rescue his family.
The pair sneak, fight and shoot their way up through the corporate building, passing by the huge tanks where human clones are grown. When he finally confronts Drucker, he gets shocking news. He finds that HE is the clone, not the other Adam. Nevertheless, he continues to fight in order to get his wife and daughter to safety. Drucker is fatally wounded in one encounter, and begins preparing a replacement clone.
Adam shows up before it is ready, and Drucker is forced to activate it anyway. The half-completed, malformed clone is revealed to be as selfish and self-centered as the original Drucker, but Drucker still gives his all to keep the new Drucker clone alive. Refusing to give up, both die in the end. His wife and daughter are rescued without ever seeing the two Adams together, so they do not have to know what has happened. With the real Adam's permission, the clone Adam goes home and says goodbye to "his" wife and daughter, knowing he will never see them again. The real Adam waves goodbye as the Adam clone heads to Argentina, taking one of the company helicopters to start a branch office and a new life..