Sakura watches a boy burn people alive with blue fire from a bus window, then he shows up at her school the next day like nothing happened. The mystery of what he actually is, what he's actually doing, and whether she should be terrified or trust him never fully resolves, but the show doesn't care because it's too busy having fun with the contradiction. Government-sanctioned teenage assassins with elemental powers and a girl caught between turning him in and falling for him is the entire appeal, and the show knows exactly how dumb and entertaining that is.

Sakura witnesses something impossible: a boy standing calmly while blue flames consume people around him. The next morning, she returns to find the scene completely erased, no bodies or evidence remaining. When a transfer student named Rei appears in her classroom, she recognizes him instantly. At school he seems kind and quickly wins people over, yet he wears one black glove on his left hand and lives by a code that answers evil with evil. Rei is a Code Breaker, a person who officially doesn't exist. Armed with mysterious powers, he pursues justice on the principle of an eye for an eye. Sakura, a cheerful girl who is also a skilled martial artist, becomes convinced that his path is wrong, and she won't accept that killing is ever right. Determined to reach whatever humanity lies beneath his cold exterior, she stays close to him in hopes of changing his ways before anyone else gets hurt. What unfolds mixes action, comedy and the supernatural, as Sakura's firm sense of fairness runs up against Rei's ruthless idea of justice and she sets out to prove that even a killer can be saved.