At first, Lilly is an overindulged tween who gets what she wants, since she has a "gift" for manipulating her dad, Henry.
Her mother Jill and brother Brandon, are often frustrated by Lilly, as they fear she is spoiled, and in need of self control.
Things begin to change when "Lilly D." (The "D" being short for Deluxe) a life sized "Really You" doll, is purchased for the precocious tween.
Soon random "accidents" begin to happen; which Lilly is quick to blame "Lilly D." for, as Lilly was told by another girl who had been invited to Lilly's house, that "Lilly D. hates her."
While Jill dismisses the claims, scolding Lilly in the process; her brother begins to suspect that Lilly D. is "more than just a doll."
Eventually, Brandon learns the truth about "Lilly D."
She is trying to replace her "human doppelganger", as Lilly D. is "evil and she wants to be human", according to the doll maker who created "Lilly D.", yet was ignored when she told the "Really You" company, to destroy the doll that eventually became "Lilly D."
After a trip to the doctor, as Lilly had fallen ill, for no apparent reason; "Lilly D." reveals to Lilly she is alive, by smashing a vanity table mirror. Lilly's protests to Jill are ignored once again; as her mother walks away carrying Lilly D., she remarks "I wish Lilly D. was my daughter, instead of you", before entering the bathroom for a few moments of relaxation.
Lilly soon starts going through a transformation, in which she is becoming Lily D. herself. Now almost completely doll, watches with one human eye that Lily D. is now Lilly, who explains if she gets Lilly's mother's love, she will be human forever. She then closes Lilly's eye and she is now passed as Lily D.
The "new Lilly" is a total reversal of the "old Lilly." Sickeningly sweet, and polite.
During dinner that evening, she was praised for her new, unselfish behavior. Unseen by the family, the "real Lilly" sheds a tear, which the "faux Lilly" wipes away, before anyone could see that her "mother upset the doll."
"Lilly" dismisses her father's suggestion that some new "accessories for Lilly D." are in order, by saying that she is "over dolls", but she is really trying to get rid of the real Lilly.
After dinner, as well as doing the dishes; "Lilly" took "Lilly D." out to the trash container, to remove her from the picture. Before going back inside,"Lilly" told the real Lilly, as an afterthought; "I always hated you."
Lilly D's scheme begins to unravel, as Brandon discovers the switch. After telling Jill to not let "Lilly D. be taken by the garbage men", he gives chase to his "sister", as Lilly's mother discovers the truth as well; the doll she holds; is the real Lilly.
Lilly is soon back to her human form when her mother realizes she should have never loved Lily D. more than Lilly, and Lily D. is now a doll again. She gets up in the middle of the street, but before she can do anything she is hit by a garbage truck.
The episode closes with "Lilly D." being found by two girls who bicker over who will get the doll. The girls run away in terror as Lilly D. grabs one of their ankles, indicating she is still very much "alive."