AI: The Somnium Files is an adventure game developed and published by Spike Chunsoft, which released the Nintendo Switch in September 2019. It is set in Tokyo and follows detective Kaname Date, who investigates serial killings and enters suspects' memories to extract information to help solve them.
AI: The Somnium Files is a mystery adventure game that is split into two types of sections: investigation and somnium parts. The story progresses through branching paths leading to multiple different endings, based on choices the player makes; the player only gets a full view of the story by playing through each route. Rather than starting over from the beginning several times to see different paths, the player can skip directly to the branching points using an in-game flow chart.
During the investigation parts, the player character, as detective Kaname Date, uses his artificial intelligence unit Aiba's functions, such as X-ray vision and zooming, to further the investigation in the real world. During the somnium parts, Date uses a machine called the Psync Machine to enter the memories of suspects or key witnesses who do not want to share information with him, in the form of dreamscapes called "somnium", to find clues and extract the information by solving puzzles. The puzzles involve examining objects and choosing an action to perform with it, and solving mental locks – obstacles standing in the way of clues – allowing the player to sink deeper into the character's subconscious. There is a six-minute time limit before the dreamscape collapses, and as actions take time to perform, the player must be careful in deciding what to do. Within the somnium parts, Aiba takes on the form of a humanoid and acts as the player avatar.
Kaname Date is called to a murder scene at Bloom Park in the quarantined Kabasaki district, where Shoko Nadami's corpse is found tied to a carousel horse with her left eye removed. Searching the area, Date finds Shoko's daughter Mizuki in the carousel's central column, clutching the murder weapon and suffering from aphonia. Date investigates the phone call made to the police about the scene, leading to his friend Renju's company Lemniscate as well as Ota and Iris. At the end of the day, it's decided for Date to psync with Mizuki to learn more about what happened and hopefully cure her aphonia.
Depending on choices made in Mizuki's somnium (and in subsequent somniums), the game branches off in different and often conflicting ways that lead to the player learning more clues. As the player progresses through the various branches, they are eventually able to piece together a lot of the game's backstory and how it ties into the present.
18 years prior to the game's events, Manaka Iwai was the mistress of Congressman So Sejima, eventually giving birth to a girl. After meeting him at the docks, Manaka was repeatedly stabbed in the back by So's psychopathic son Saito and buried by the Kumakuras after Rohan removed her right eye. Following a GPS tracker, Renju and Hitomi were able to dig her up shortly after and kept her body frozen in the Okiura cold warehouse since then. Manaka's baby was raised by Hitomi, who chose the name Iris.
6 years prior to the game's events, Hayato Yagyu was a police officer started killing criminals after realised they would quickly walk free, leaving nothing for the police to find. Rohan discovered it and forced the officer to doing the same for the Kumakuras, giving him the codename "Falco". One such job led him being chased after taking a bullet to the stomach. Hitomi found him, nursed him back to health and the pair started a relationship, becoming a father figure to Iris. One day, Rohan broke into their home and attempted to shoot Falco, but Hitomi moved into the path of the bullets (and lost the nerves in her right arm as a result). Falco was arrested for his past events, becoming #89.
In addition, the player learns a state secret: The reason for the six minute limit left when in Somnium is because spending too long in Somnium can cause the conciousness and memories of both the psyncer and host to switch, causing them to change bodies. The player also finds the prototype Psync machine, in the abandoned chemical plant in Kabasaki where the explosion took place.
Date had been incapacitated by Falco at the disused chemical plant, after discovering Renju dead. After waking up strapped into the prototype Psync machine, he realises #89 in front of him is actually Saito. Saito laid out what happened: As Shoko, he had lured Renju to the chemical plant, swapped bodies and left Shoko's body in Bloom Park. Next, he had attempted to lure Iris but Date's warning earlier in the story prevented that. After getting into a serious car accident while in Renju's body, Saito had to improvise - When he had secretly heard from Pewter that #89 was being sent to ABIS, "Renju" asked Pewter to help #89 escape. With Rohan (as #89/Falco) and Saito (as Renju) reunited, the two swapped bodies, causing Rohan's consciousness (now in Renju's body) to die due to Renju's injuries. Saito then used Falco's body to lure Iris to the plant, since she had been looking for him since, which in turn lured Hitomi and Date to him.
With all this explained, Saito forcibly psync'd with Date, exchanging bodies in the process. As Saito celebrated and Date went unconcious, he remembered what happened six years ago - Wanting to leave the Kumakuras' service, Rohan had demanded Falco do one last job: Killing Hitomi and Iris due to their connection to Manaka's murder. Knowing someone else would've killed them if he failed to do so, he confessed his past to Boss who suggested swapping bodies with Rohan so he could call the hit off. During the Psync between himself and Rohan, Falco also discovered that the original Cyclops Killer was Saito with Rohan removing their victims' right eyeballs post-death. However, Rohan escaped and went to Hitomi's house to kill her - Falco in Rohan's body arrived to save Hitomi, but inadvertently shot her as she defended "Falco". Boss and the police arrested Rohan while still in Falco's body, becoming #89.
While still in Rohan's body, Falco went to see Saito to get a confession for the Cyclops Killings. However, Saito saw through the deception and interrogated Falco, learning of the Psync technology and the ability to change bodies. Stealing the prototype Psync machine, Saito swapped bodies with Falco. However, Falco escaped before the Psync process finished, taking Saito's body but leaving his memories behind. Found wandering the streets by Boss, she created a new identity for him: Kaname Date. Because of Falco's past, the MPD covered it up effectively erasing Hayato Yagyu from history. Saito in turn stayed in Rohan's body in a secure hospital for five years, before somehow swapping bodies with Shoko Nadami. The swap caused Shoko (in Rohan's body) to jump off a roof to his/her suicide, while also starting the chain of swaps encompassing the New Cyclops Killings.
Before he could force Date to watch Hitomi and Iris die, Saito is suddenly ambushed and chased off by Moma, who Aiba had called for help. Following further into the plant with a rifle, Date sees Iris held at gunpoint by Saito. Just as Saito is about to fire, he is similarly ambushed by Mizuki and Ota, the latter untying Iris. The group chase Saito to the top floor, where he has Hitomi at gunpoint. Facing a dilemma of how to save her, Date is called by Aiba who suggests her self-destruct mechanism, which would kill Saito but also herself. Date invokes it with great reluctance, causing Saito's head to explode and saving Hitomi. Mourning Aiba's sacrifice, Date forgets his old identity and reaffirms himself as Kaname Date.
Three months later, Iris is recovering from nanotech surgery to remove her brain tumors, unknowingly paid for by the MPD in return for Date's friends keeping what they know on the Old and New Cyclops Killers secret. Iris asks Date to take her to the game's various characters. While talking with Boss, Date is unexpectedly knocked out by her and wakes up in the Psync room with Pewter who reports on his incarceration. So is less angry now that Date is no longer reminding him of Saito, and reveals he is no longer a politician, his agreement for Saito to disappear was his suspected leverage against the MPD, and that the chemical plant explosion was Saito's plan so that So could turn Kabasaki into a casino district. Mizuki now owns Lemniscate due to inheriting Renju's ownership but is still arguing with Date. Ota now works multiple jobs, but is still exaggerating his heroism to anyone listening. Mayumi now has medication to help her delirium and has resolved her past animosity towards Iris. Moma has spread his love for A-Set to the rest of the Kumakuras. Since psyncing isn't known to the public, everyone believes that Date needed plastic surgery and it unfortunately made him look like Falco.
Finally, Iris takes Date to the warehouse district where she shows him a surprise: Aiba back from the dead, rebuilt using the Wadjet System and implanted by Pewter while unconscious at ABIS. The game ends with Iris leading a dance number featuring most of the surviving characters.