The central twist of "W: Two Worlds" is that the webtoon world of "W" and the real world are parallel dimensions that can intersect. The protagonist of the webtoon, Kang Chul, develops his own consciousness and is able to pull Oh Yeon-joo, the daughter of his creator, into his world. The narrative's true antagonist is not just a simple killer, but a plot device that also gains sentience. This faceless killer was created by Oh Seong-moo without any identity or motive, simply to provide conflict for Kang Chul. This lack of context allows the killer to develop his own will: a desire for an identity, which he achieves by stealing Oh Seong-moo's face and agency.
A major plot turn occurs when Kang Chul, to end the chaos, convinces Yeon-joo to reset his memories, making everything that happened between them a mere dream in the webtoon. This erases their relationship and seemingly puts the story back on track, but the connection between the worlds remains. The second half of the series deals with the fallout of this reset, as Yeon-joo is now a wanted fugitive in a world where her husband doesn't remember her.
The finale reveals the ultimate rule of this universe: the story must have a definitive ending. To save everyone, Oh Seong-moo, who has been battling the villain for control of his body, makes a final sacrifice. Trapped in the webtoon world, he uses the last of his power to draw the villain's death and ensure Kang Chul's survival and freedom. He then fades away, as his role as the story's final antagonist is complete. Kang Chul serves a prison sentence in the webtoon world before the story officially 'ends,' allowing him to cross over permanently to the real world and reunite with Yeon-joo. The 'happy ending' for the couple is thus predicated on the tragic, sacrificial 'sad ending' for Yeon-joo's father.