OPUS revolves around a manga author named Chikara Nagai, who is working on a dystopian sci-fi manga entitled ''Resonance''. As the deadline approaches for him to finish the final chapter of his long-running work, he comes up with an ending in which major character Lin kills the antagonist but loses his own life in the process. However, after drawing the final page, Chikara finds that a tunnel has appeared on his paper, and Lin himself has stolen the drawing, intending to stop his own death by preventing Chikara from finishing the manga.
What follows is a meta adventure story in which Chikara has to travel through his own story, convincing the characters that he is responsible for their existence and battling against the forces of the antagonist of Resonance, The Mask.
This manga provides examples of:
- A God Am I: The Mask, obsessed with complete control, ultimately strives to be this.
- A God I Am Not: Satoko and Mei address Chikara as "God," but Chikara insists he is merely a manga artist.
- Chekhov's Gun: The oversized ventilation grate drawn by Chikara's assistant is what he falls through when he first enters the world of Resonance.
- Da Chief: Satoko's boss, who literally carries the title. (Well, minus "Da.")
- Faceless Masses: The minor extras in the background of Resonance. Being unimportant, not much detail was put into them by the artists - which makes them very strange-looking to the people inside the manga when they approach them.
- Creator Cameo: Satoshi Kon himself appears in one panel wondering aloud what dialogue is doing in the manga that he didn't write. The unfinished final chapter has Chikara appearing in the real world to visit Kon and his wife after the magazine which carries Opus has been shut down.
- Gainax Ending: It makes sense within the story's rules, but it's still unexpected, as it revolves around Satoshi Kon himself, the cancellation of the manga, and eventually Chikara learning that he's just a manga character too.
- God Is Evil: Satoko calls God an asshole, first during her confrontation with The Mask, and then when she realizes how horrible the real world is.
- Hot-Blooded: Don't let Satoko's cute appearance fool you.
- Mind Screw: Although light compared to Kon's later works, it's still there.
- Most Writers Are Writers: Justified, as the story is literally about a manga.
- No Fourth Wall: In-universe, this is the basis for the plot, as more and more characters discover this fact.
- Rage Against the Author: The catalyst for the whole plot is Lin being dissatisfied with being killed off for the sake of a cheap Twist Ending. Eventually the characters have to work together with their author in order to combat the antagonist.
- Real Life Writes the Plot: The final chapter is all about the magazine that Satoshi Kon is publishing OPUS in getting cancelled, and Kon's moving on to write movies instead of manga. It's a case where real life is the plot, until the later parts of the chapter.
- They Killed Kenny Again: Satoko dies twice. First when she is shot by the brainwashed Chief and then when she is erased from existence. The governing Kon-esque laws of the story bring her back to life shortly afterwards though.