In the 22nd century, humanity faces the devastating consequences of severe global warming which has resulted in melting polar ice caps and flooded coastlines, drastically reducing the human population. Amidst this dystopian landscape, a new class of robots known as Mecha emerges. These advanced humanoids are designed to mimic human emotions and thought processes. David, a prototype created by the Cybertronics corporation in New York, is uniquely constructed to resemble a human child and was specifically programmed to exhibit love towards his human owners. He is introduced to employees Henry Swinton and his wife Monica for testing.
The Swintons' biological son, Martin, is in suspended animation due to a rare illness, and although Monica initially fears David, she gradually grows to love him once she activates his imprinting protocol. This irreversible bond causes David to love Monica as any child would love their parent. Alongside him is Teddy, a robotic teddy bear, who becomes David's protector and confidant.
As fate would have it, a cure becomes available for Martin, who is brought home, leading to a tense sibling rivalry with David. Martin tricks David into cutting a lock of Monica's hair in the middle of the night, which deeply upsets the parents when they discover it. During a pool party, a friend of Martin’s inadvertently activates David’s self-defense program, resulting in a dramatic incident where David, in panic, pulls Martin into the pool. While Martin is saved, Henry begins to fear that David's capacity for love also includes the potential for hatred.
Convinced of the risks David poses, Henry persuades Monica to take him back to Cybertronics for destruction. However, Monica instead chooses to abandon David in a forest, leaving him and Teddy to fend for themselves as unregistered Mecha. Captured soon after, David is taken to an anti-Mecha Flesh Fair, an event where outdated Mecha are destroyed for entertainment. Here, David pleads for his life, showcasing emotions that sway the crowd, prompting him to escape with Gigolo Joe, a male Mecha played by Jude Law, who is evading the authorities on charges of murder.
Together, David and Joe embark on a journey to find the Blue Fairy, a figure from the story of Pinocchio, whom David believes can make him a real boy so Monica can love him and take him back home. Their adventures lead them to Rouge City, where they consult a holographic entity named Dr. Know before making their way to the submerged ruins of New York City in a stolen Amphibicopter. Here, David meets his creator, Professor Hobby (played by William Hurt), who reveals that the journey was a test of David's love, and many more copies of him are being produced, leaving David heartbroken and questioning his uniqueness.
In despair, David attempts to end his own life but is rescued by Joe. David, striving to reach the Blue Fairy, encounters a submerged statue, believing it to be the embodiment of his dreams. Tragically, as they become trapped under a collapsing attraction, David's wish persists unyieldingly until a new ice age takes hold, leaving him to wait for thousands of years.
Fast forward two millennia, with humanity now extinct and New York buried beneath glacial ice, advanced Mecha discover David and Teddy. Recognizing their connection to living humans makes them special. Once revived, David approaches the decayed Blue Fairy statue, which crumbles at his touch. The advanced Mecha inform David that they cannot grant his wish to become human. Still, at David’s insistence, they use the DNA from Monica's hair to reconstruct her, but the process allows her to live only for one fleeting day.
During their ephemeral reunion, Monica reassures David of her unconditional love before drifting into her final sleep. David, filled with joy, lies beside her and closes his eyes, embarking on a journey to "that place where dreams are born," as Teddy watches over them both in their serene moment together.
