The "99.9" refers to the 99.9% conviction rate in Japanese criminal cases, and the lawyer who solves the remaining 0.1% of acquittals is the protagonist of this film.
Hiroto Miyama works with rookie lawyer Honoka Kono on a team specializing in criminal cases for Madarame Law Firm. Atsuhiro Sada has recently become director. One day, the firm receives a request about a case that took place 15 years ago—concerning a poisonous wine incident.