The Switch is a comedy about a woman who wants to have a baby before her biological clock runs out. Jennifer Aniston plays Kassie, a woman who has given up on finding the perfect mate and decides to use a sperm donor as a way of becoming pregnant. Kassie's best friend is a man named Wally who has been in love with Kassie for some time. Complications arise when the sperm donor Kassie chooses attends a special party thrown by her closest friends. The donor turns out to be a handsome together type of man, which is a personality in contrast to Wally's often neurotic behavior.
The party Kassie's friends decide to give her is more of a ritual which includes the actual act of insemination. The donated sperm is left inside a cup in the bathroom. During the party, Wally tries to drown his jealousy of the handsome sperm donor by becoming very drunk. He stumbles his way to the bathroom where he accidentally knocks over the cup of sperm. Taking matters into his own hands, he decides to replace it with his own. He ends up passing out and forgetting the whole experience. Kassie goes through with her plans and uses the sperm which does prove to be effective.
After Kassie gets pregnant, the story starts to move forward at a faster pace. She develops a relationship with the donor, believing he is the father of her son. As the boy grows older he begins to exhibit strange neurotic behaviors which are very similar to the ones Wally has. While visiting Kassie one day, Wally suddenly has a revelation where he remembers what he did in the bathroom the night of the party for Kassie's insemination. All of a sudden, everything about the boy makes sense to him. He knows he is the father, but does not know how to go about telling Kassie what he did.