An alternate retelling of the events of A Certain Magical Index, the fic details the adventures not only of Touma Kamijou, but Naoki Kashima (aka the Demi-Fiend) after the latter crash-lands in Academy City with foggy memories of what happened to him, and his power drastically reduced. Becoming Touma's roommate, Naoki tags along with the unlucky esper on his adventures, becoming an unpredictable force of chaos in his already out of control life.
A Certain Metaphysical Experiment contains examples of:
- Always Someone Better: Kanzaki Kaori may be a Saint, a holy demigod of destructive might that makes Magical Nukes look tame, but even in his weakened state the Demi-Fiend considers her less noteworthy than creatures he squashed at the beginning of his career.
- Berserk Button: In keeping with being the Chaos King, Naoki is sent flying into a rage by the smell of Lawfulness. The only reason he tolerates Index's presence is because her lack of power makes her scent much weaker. Kanzaki, on the other hand...
- Big Bad: Whoever or whatever is currently manipulating Lucifer.
- Color-Coded for Your Convenience: The Demi-Fiend's Power Tattoos are an indicator of his health. They flash a dull red when injured, and glow green-blue when healthy.
- Compelling Voice: Zig-Zagged with Naoki. His voice immediately draws the attention of those around him and has a degree of authority, and those of a more chaotic persuasion find it very pleasing, but more neutral individuals find it unsettling, while lawful listeners become terrified. The same effects occur with eye contact.
- Deadpan Snarker: Naoki has some small signs of being one, coupled with occasional hints of being a Troll, but his emotions are fractured so it's hard to tell.
- Humanoid Abomination: The strange individual standing behind Lucifer. A being so revolting in its wrongness that even the Chairman of Chaos finds it disturbing, and one that inspires an Oh, Crap! reaction in him.
- Index also certainly considers the Demi-Fiend to be one.
- Older Than They Look: The Demi-Fiend looks like he's in his late teens, but as an entity that once transcended time itself, his age is technically infinite.
- No-Sell: Subverted in Touma and Naoki's initial meeting on the bridge. Naoki expects to be able to No-Sell Mikoto's lighting with no problem, but is unable to (due to a combination of Imagine Breaker and his own reduced power) and gets knocked out.
- Not Distracted by the Sexy: Naoki doesn't react in the slightest to the sight of Index's robe getting negated by Imagine Breaker.
- Time Stands Still: Done by Lucifer in order to keep Naoki from destroying an alleyway in rage.
- Unstoppable Rage: A side-effect of Naoki's newly-restored humanity and inability to control his emotions, coupled with the scent of Lawfulness.