Mochizuki General Hospital boasts some of the most well-trained nurses in town. But though these ladies spend much of their day battling sickness, their after-hours are spent fighting a very different sort of disease... Under the leadership of the hospital chairman, a handful of staff members and local teenagers form a group of mercenary assassins, targeting the "cancers" of society and excising those individuals before their wickedness spreads.
A triage is when several people are injured and a system is employed where five different label colors are applied based on the severity and urgency of the injuries, and then the people are treated based on that system. Black is the closest color to death. The illegal organization Black Label, applies these labels of death to criminals that eat away at society, just like malignant tumors, and run operations to stop them.
Mikami Arashi, a high school student, has excised his first malignant tumor, and is now an official member of the illegal organization, Black Label. He took on the role of executioner because of what happened nine years ago during an act of terrorism against an international medical conference. And now Black Label moves to save a victim which has been kidnapped right after they saved them the last time.
Kiba Mikoto, a student at the same school as Arashi, fights evil along with him part of the organization, Black Label. Mikoto runs into a mysterious transfer student, Hizaki Chikage. Mikoto learns a lot about Chikage, and the two of them get very close. Meanwhile, a string of incidents happen where citizens are being burned to death...
Mikoto knows of the existence of a malignant tumor and rushes off alone to excise it, without waiting for the rest of Black Label. However, she runs across people taking a new narcotic, and someone else she didn't expect...
She can sing, dance, and act. A top idol, Nashida Oriha, is also a member of Black Label. She spends every day riding the world of evil. She's slated to help run a live broadcast celebrating a new TV studio, but things go wrong when the studio is targeted. How will Oriha get through this ordeal?
It's time for the show. Bad guys have hijacked the TV studio and taken the idols hostage. They've asked for a large some of money, and if they don't get they, they're going to force their hostages out of the 18th floor down to the ground below. Meanwhile, Oriha manages to slip away, and learns of their true intentions...
Sagiri Yuko, Tsurugi Miki, and Sayo Hitsugi, Ampule Zero, and Arashi, Mikoto, and Oriha, Ampule One, go to investigate a cargo ship that had arrived at Tobioka Port. Twins show up calling Hitsugi "senpai", but what is the true reason they want to pick a fight with her...
Hitsuji loses herself and goes on a rampage, and her friends, other members of Black Label, find themselves raising arms against her. However, Arashi can't bring himself to fight her for real, as he doesn't want to hurt her, but Yuko decides to execute the Triage...
Black Label rids the city of evil by severing the malignant tumors that have made their home there. Suzue Konomi, a detective, has looked into their organization, and finds herself kidnapped. Her assailants true objectives are a mystery, and more than just one group is after her. Will Black Label be able to sever the evil?
Black Label goes over the information they have thus far in order to figure out what's going on. It's clear there's a new malignant tumor out there. Just what does the Triage mean to Arashi and the others, and why have they chosen this way of life?
