Nanao 'the unluckiest assassin in the world' has been hired to deliver a birthday present to a guest at a luxury Tokyo Hotel. It seems like a simple assignment but by the time he leaves the guest's room one man is dead and more will soon follow. As events spiral out of control as it becomes clear several different killers, with varying missions, are all taking a stay in the hotel at the same time. And they're all particularly interested in a young woman with a photographic memory, hiding out on one of the twenty floors. Will Nanao find the truth about what's going on? And will he check out alive?