Masami Aomame is a 30-year-old woman living in Tokyo in 1984. A fitness instructor with a well-honed understanding of the human body, she is also an assassin who murders domestic abusers by inserting a tiny needle into the back of her targets’ necks, making the deaths look like the result of natural causes. Her missions are funded and organized by Shizue Ogata, a wealthy woman in her 70s who is referred to as “the dowager.”

At the start of the novel, Aomame is in a taxi stopped on the expressway in heavy traffic. She is on her way to kill a wealthy businessman who savagely beats his wife. As Leoš Janáček’s Sinfonietta plays on the radio, the driver tells Aomame that the only way she will make it to her appointment is if she exits the taxi and enters an emergency stairway that leads to street level. From there, she can take the subway to her appointment. As Aomame leaves the car, the driver warns her, “[Y]ou’re about to do something out of the ordinary. [...] And after you do something like that, the everyday look of things might seem to change a little. [...] But don’t let appearances fool you.