The year is 2027 in MegaTokyo, six years before the Knight Sabers will make their debut. Boomers (artificial humans) are still a relatively new advancement, and the implementation and integration of boomers into society is still a bit buggy -- sometimes fatally so. Whenever a boomer incident occurs, though, there is the Advanced Police, a special force trained to deal with boomer crimes. Leon McNichol is a rookie in the AD Police, and is just starting to become exposed to the horrors and tragedies one finds every day in MegaTokyo. He and his veteran partner, Gina Marceau, slowly learn about the ever-fading line that separates man from machine.

The new order of ACCA will soon hold its first-anniversary celebration. To ensure every district is prepared for the festival, Jean Otus—acting as a representative for ACCA's inspection department—is called to travel across the Kingdom of Dowa and help with the arrangements. However, he feels that in times of peace, he cannot relate to his subordinate Parrot, who believes that another betrayal against Dowa is near certain. Meanwhile, work leaves Jean no time to dedicate to his little sister Lotta and the unexpected predicament she seems to be hiding from him. Their best friend, Niino, decides to involve himself and looks for a way to support Lotta. Although Jean believes that Dowa is in no immediate danger, he must ease Parrot's suspicions before the festival and wait for Lotta to open up to him on her own.

ACCA—a national body of the kingdom of Dowa that provides public services to the citizens of the country—was established as part of the peace settlement between the king of Dowa and the 13 states of the country during a revolt. One hundred years later, Dowa is in a period of unprecedented peace, due in part to the ACCA system. However, rumors of a coup d'état start to surface. Jean Otus, the second-in-command of the inspection department of ACCA, is charged with inspecting all 13 state branches. What will he discover as he performs his audit? Intriguing and mysterious, ACCA: 13-ku Kansatsu-ka is a politically-themed mystery that reveals a world of diverse cultures and lifestyles, with intricate connections between its characters, as the truth of the coup d'état slowly unfolds.

Young Mabel West is the daughter of mystery writer Raymond West, who wants her to lead a normal life. Rebelling against this, Mabel wants to be a great detective, and sets out for London to become assistant to none other than Hercule Poirot, the great Belgian detective who resides there. She finally wins the reluctant approval of her father, and embarks on an exciting life of mystery and suspense—his only demand being that she occasionally spend some time with her great-aunt, Jane Marple, in the small village of St. Mary Mead.