When 14-year-old middle-schooler Tsugumi Harudori discovers she is a weapon, she flies to America to enroll at DWMA. There, she meets the absent-minded Meme Tatane and the patrician Anya Hepburn, both of whom are meisters that are also new enrollees.

Tsugumi and the others get a bad impression from the Thompson sisters, Liz and Patty, who are working at Deathbucks whilst on probation, having spent most of their childhood living on the streets of Brooklyn. When they run into the sisters later that day, Anya prepares to stand against them with Tsugumi, but is outmatched by their ability to both transform into pistols.

Whilst the rest of Death City celebrate Halloween with a Battle Festival amongst the EAT students, Kim and Jacqueline are tasked with watching over Tsugumi, who had been grounded for her actions. Meanwhile, as Akane and Clay escort Anya towards the airport, the city is suddenly overrun by NOT students turned into Traitors, which the EAT students have to fend off.