Dai Gyakuten Saiban takes the Ace Attorney series to the past. The player controls Ryūnosuke Naruhodō, a Meiji-era ancestor of Phoenix Wright who is studying to become a defense attorney. He makes a name for himself after a trial in which he successfully represents himself, and ends up accompanying fellow student Asōgi Kazuma in his placement in England to study the British legal system. The two of them are joined by Susato Mikotoba, a legal assistant and the daughter of Naruhodō's legal mentor. Naruhodō ends up instantly in over his head, forced to defend clients in the British legal systems while getting help from strange allies - including Sherlock Holmes himself.
Gameplay follows the traditional Ace Attorney style, with a few new features. Like in Professor Layton VS Phoenix Wright: Ace Attorney, cross-examinations can sometimes involve multiple witnesses. Courtroom sessions are now held in front of juries, who are empowered to stop proceedings and issue a verdict at any time. Luckily, Naruhodō is allowed to provide closing arguments which can result in overturning verdicts. Outside of trials, Naruhodō and Holmes can collaborate on deductions that lead to new breakthroughs using the new "Deductions of Sherlock Holmes" feature.
Like other Ace Attorney games, the plot is divided into five cases:
The Adventure of the Great Departure: Naruhodō is accused of the murder of a visiting British medical professor, Dr. Watson, and is forced to defend himself in court.
The Adventure of the Unbreakable Speckled Band: A passenger is killed on board the steamship making the journey from Japan to England in a locked room mystery. Naruhodō finds himself accused of the crime again, and is forced to clear his name. Luckily, Sherlock Holmes turns out to be on board the ship and offers his help - whether Naruhodō wants it or not.
The Adventure of the Runaway Room: After arriving in England, Naruhodō is forced to take on a trial to earn his keep. In another locked room mystery, the business magnate Cosney Megundal is accused of murdering a fellow passenger on a horse-drawn cab while it's making the rounds.
The Adventure of Clouded Kokoro: Naruhodō defends Japanese exchange student Soseki Natsume, accused of stabbing a stranger in the street.
The Adventure of the Unspeakable Story: After the owner of Holmes's favourite pawn shop is murdered, the defendant ends up being Sherlock Holmes himself.