Dai Gyakuten Saiban: Naruhodō Ryūnosuke no Bōken

Taking place near the end of the 19th century and the dawn of the 20th century (known in Japan as the Meiji Period, in Britain as the Victorian era), Dai Gyakuten Saiban focuses on Ryūnosuke Naruhodō, an English student at Teito Yūmei University and an ancestor of primary Ace Attorney series protagonist Phoenix Wright. He is described as a character with a strong sense of justice, but who easily ends up in danger.[4] Joining Ryūnosuke is Asōgi Kazuma, his friend and law student. He is studying to become a defense attorney, but is described by Capcom as being "more than qualified to be one" already. As the modern judicial system was new at the time in Japan, Asōgi aims to complete his studies in Great Britain.[1] Ryūnosuke and Asōgi are aided by Susato Mikotoba, a judicial assistant. She is described by Capcom as a yamato nadeshiko (a personification of the image of the ideal Japanese woman), a progressive dreamer, and a lover of foreign mystery novels.[5][6]

Upon traveling to England to further his studies, Ryūnosuke meets and befriends world-famous British detective Sherlock Holmes.[7] Unlike the literary version of the character, Holmes often misjudges clues left around the crime scene. Accompanying Holmes is his assistant Iris Watson, a ten-year-old prodigy and the daughter of the deceased Dr. John H Watson.[7]

The first chapter, The Adventure of the Great Departure, takes place in Meiji Japan, and has Ryūnosuke get put on trial for the murder of a Japanese university teacher, Dr. John H. Watson. He is represented by Asōgi, while the prosecutor is Taketsuchi Auchi, the Meiji Period ancestor of recurring character Takefumi Auchi (Winston Payne in the English localizations). With help from Susato, Ryūnosuke and Asōgi prove that the real murderer is Jezail Brett, an English exchange student who had been studying under Watson at the university, though her motive is left a mystery and she was to be sent to China due the consular courts. Following the trial, Asōgi leaves Japan to complete his studies in Great Britain, but has Ryūnosuke travel with him as a stowaway.

In the second chapter, The Adventure of the Unbreakable Speckled Band, Asōgi hides Ryūnosuke in his baggage and then in his room in order to bring him along to England for Asōgi's debut in the courts. However, while Ryūnosuke is asleep, Asōgi is found murdered within his cabin, and now exposed stowaway Ryūnosuke is once again blamed for the murder. Working with Susato and eccentric passenger Sherlock Holmes, Ryūnosuke discovers that the real killer is Nikomina Borschevitch, a world-famous Russian ballerina who fled Russia, and had unintentionally killed Asōgi by pushing and breaking his neck on his bed's wooden knob after fearfully assuming that he was going to turn her in to the captain (in truth he attempted to introduce her to her fellow stowaway Ryūnosuke as an act of trust and similar circumstances). After the incident is settled, Ryūnosuke persuades Susato for him to take Asōgi's place as a representative lawyer in Britain, which she agrees to and begins to heavily tutor him on English Law for the remainder of the journey.

The third chapter, The Adventure of the Runaway Room, has Chief Magistrate Lord Hart Vortex suddenly assign Ryūnosuke and Susato to defend a philanthropist Cosney Megundal, who has been accused of murdering a bricklayer named "Thrice-Fired" Mortar Milverton within a moving omnibus, shortly after they arrive in Britain. They go up against Barok van Zieks, a legendary British Prosecutor known as the "Grim Reaper of the Old Bailey" and are introduced to the British legal system, which uses a jury of six people to decide the verdict. During the trial, a smoke bomb is let off, forcing an evacuation of the courtroom. After court resumes, Ryūnosuke and Susato cross examine the newly discovered witness, Gina Lestrade and discover evidence proving that Mortar had been killed on the carriage roof, though Zieks claims that this evidence had been forged after the smoke bomb had gone off. He is unable to prove this, and Megundal is subsequently acquitted, though Ryūnosuke remains unsure of Megundal's innocence. Shortly after the trial, Megundal is murdered in a fire.

In the fourth, The Adventure of Clouded Kokoro, Ryūnosuke and Susato take on the defense of the soon to be famous Japanese novelist Natsume Sōseki, who is currently in London studying English as a nervous exchange student, and has been charged with aggravated assault following a stabbing in the street. During the trial, Ryūnosuke proves that the stabbing was actually the result of a domestic disturbance between landlord John Garrideb and his wife Joan, who had thrown a knife at her husband, only for the knife to fall outside their window and unintentionally hit the victim. Sōseki is declared not guilty and explains that he is going to go back to Japan, no longer as nervous of the public as he was before. Sherlock invites Ryūnosuke and Susato to live with him and Iris at 221B Baker Street and set up their law office in their attic.

In the fifth and final case, The Adventure of the Unspeakable Story the mystery of the third case is returned to, as a bigger game at hand is revealed through the murder of a pawnshop owner, Hutch, which Gina is accused of. It turns of one of the jurors' acquaintance, a Telegraph station technician named Robert Crogley, was the son of Mortar Milverton, who Megundal did kill, and had the baliff set the fire that killed Megundal. Crogley killed Hutch to steal a device with government secrets which had resulted in Megundal murdering Milverton. After the case is solved, Susato heads back to Japan after learning that her father is supposedly really sick.

Unlike previous Ace Attorney games, both characters and environments are fully made in 3D.[20] Like Takumi's earlier game Ghost Trick: Phantom Detective, spotlights light up characters while they dance; this was something that had not been done in previous Ace Attorney games.[15] Motion capture of the former Takarazuka Revue actress Shiotsuki Shuu was used for some animations; as the Ace Attorney series is known for its poses and animations, this was chosen as a way to make the animation more dynamic.[21] Nuri designed the characters to make them look simple while also conveying a lot of information. He intended to make the graphics look like illustrations, and wanted to convey the feeling of the materials clothes and items from the Meiji period were made from. Characters were designed to be partially realistic, as realistic animations and facial expressions were needed for the game. Witnesses and jury members were however designed in a more stylized way, to ensure that players immediately recognize them when they see the characters sitting next to each other.[10]

Ryūnosuke was designed to have "the sharp look of a university student", which went without problems. It did, however, take a long time to design his haircut; Takumi wanted the haircut to be recognizable from Ryūnosuke's silhouette, but said that most people during the Meiji period had simple haircuts.[18] Nuri tried designing various haircuts from that time, and designed around 50 variations.[10] Susato was designed to be an elegant Japanese woman; because of the time period, it was decided that she was to wear a kimono. Takumi had several ideas for various items she could hold, but it was decided that it was best to keep her design simple,[19] as contrast to the English characters. Several different variations were made for Holmes' design, including "depressed", "cute", "adventurous", "dark", and "sleeping" variations; eventually, they used a variation on how Holmes traditionally is depicted, as contrast to his personality. Additionally, he was given a gun as contrast to Ryūnosuke's sword. Iris' design included gothic elements, as well as elements of a mad scientist, and was created to look good when appearing together with Holmes. Asōgi's clothes were designed with influences from both Japanese and Western cultures. His headband, which flutters in the wind, was something Nuri really wanted in the game, and was something they were only able to do with 3D graphics.[10] Van Zieks was designed to give off a "dark aura",[22] with elements of vampires, wolves, and fallen angels.[10]

When composing the game's music, Kitagawa used a mindset similar to that when composing for older games, trying to work within limitations to create strong music. He focused on creating catchy melodies, and went through a lot of trial and error. Takumi wanted the music to sound more "festive" than the electronic music used in earlier Ace Attorney games, so he asked Kitagawa to make use of a live orchestra, but with fewer instruments than in the previous game. He also wanted it to sound British, so Kitagawa ended up using sounds similar to chamber music. They did not think piano sounded right for the game, which led to difficulties in differentiating the instruments, as they only had string and wind instruments; because of this, they ended up introducing Spanish elements to the music. As a reference to how Holmes is known for playing the violin, they also tried using violins in the music.[9]