Ai: The Somnium Files is an extremely complicated story with multiple branching paths, so the summary will be divided into the backstory and then divided into ending paths.
Spoilers will be unmarked.
Backstory
30 years before the beginning of the game, Japanese congressman So Sejima has a son with his wife, who dies in childbirth. He names the son Saito Sejima. Throughout Saito's childhood, he develops distant and uncaring feelings toward his father due to Saito having a disorder that prevents his brain from producing oxytocin, the "love hormone". Violence is the only thing that makes Saito happy. Saito begins killing small animals, but wants to kill a human once he builds up a tolerance for the pleasure that killing animals brings him.
18 years before the beginning of the game, So Sejima has an affair with Manaka Iwai, a 19-year old who just graduated high school. Manaka becomes pregnant with So's baby. So pays for Manaka to get an abortion and cuts ties with her, but Manaka secretly carries the baby to term. Shortly after giving birth to a daughter, Manaka confronts So about the whole situation. So believes Manaka to still be pregnant when he sees her, as her belly has not shrunk yet from giving birth. He offers to pay for her to move to another country, but before the conversation gets very far, Saito reveals that he has followed his father to his meeting, and kills Manaka. So doesn't want his political career to be ruined by having his son go to jail and having people discover that he impregnated a teenage girl, so he contacts Rohan Kumakura, the head of the Kumakura branch of the yakuza, to dispose of the body. Manaka's best friends, Renju Okiura and Hitomi Sagan, witness the disposal of Manaka's body but are afraid to say anything to the police for fear of retaliation. Hitomi adopts Manaka's daughter and raises her as her own to keep the baby safe from So and the Kumakuras. Renju prepares the documentation that Hitomi needs to register the baby as her own. The baby is named Iris Sagan. Meanwhile, the two of them exhume the body and move it to an out-of-use cold storage warehouse owned by Renju's father, in order that it might be preserved, and place it into the trunk of a forklift.
8 years before the beginning of the game, a detective going by the codename Falco grows sick of not being able to stop the worst of the worst criminals, and becomes a vigilante, hunting down criminals that managed to avoid justice and killing them. The Kumakura gang discovers Falco's secret and blackmails him into becoming their assassin.
6 years before the beginning of the game, Rohan Kumakura contacts Saito Sejima with a proposal: Saito kills women, and Rohan collects their right eyes, so that he may admire the beautiful eyes of women more intimately. Saito is happy to have an excuse to murder and agrees. Rohan writes a letter to the press in an attempt to confuse the investigation, calling himself "The Cyclops Killer". Around this same time, Falco is injured during an assassination mission and is found by Hitomi Sagan. The two fall in love and Falco becomes like a father to Iris Sagan. Falco wants to leave the criminal life behind and tells Rohan Kumakura that his next hit will be his last. Rohan agrees to this, but when he hands Falco the picture for his last hit job, it is a photo of Hitomi and Iris Sagan. Falco tells his commanding officer at the police, who he simply calls "Boss", everything about his life as an assassin, hoping she can help. Boss tells Falco about a device that the secret arm of the police called ABIS uses, the Psync Machine. The Psync Machine allow a detective to investigate the dreams of a suspect, but a secondary use it has is that it can cause two users to swap bodies if used for more than six minutes. Boss brings in Rohan on unrelated charges, drugs him, and hooks him into the Psync Machine. Falco swaps bodies with Rohan and plans to use his influence as "Rohan" to tell the yakuza to keep their hands off of Hitomi and Iris. However, the real Rohan, in the body of Falco, escapes the police station and heads to Hitomi's house to kill her himself. Falco and Boss give chase, but Hitomi has already allowed Rohan into her home by the time they arrive, believing Rohan to be Falco. A firefight happens and Hitomi is shot in the right shoulder in the crossfire, leaving the arm paralyzed. Rohan is arrested and sent to prison.
Falco assumes the position of head of the Kumakura gang. He then meets with Saito Sejima in an attempt to bring in the other half of the Cyclops Killer duo, but Saito realizes that he is not actually Rohan. After forcing the truth out of Falco, Saito learns about the ability to swap minds. He orders his men to steal a prototype Psync machine from the Chiba PD, and uses it to swap bodies with Falco. Falco escapes in Saito's Body missing one eye, but the prototype Psync Machine leaves both Falco and Saito with compromised memories. Boss realizes who Falco is and uses her connections to blackmail So into pretending his son left the country, and allowing Falco to take on a new identity in his new body. She gives him the name Kaname Date. Saito, in Rohan's body, seems to have a mental breakdown and gives control of the Kumakura gang to Rohan's younger brother, Moma Kumakura.
5 years before the beginning of the game, Kaname Date is given a prosthetic eyeball with an AI named Aiba. Aiba helps Date solve cases and secretly injects him with oxytocin so that he can still feel love without murdering anyone.
4 years before the beginning of the game, Renju feels that he is failing as a father to his daughter, Mizuki Okiura, and asks Date if he will let Mizuki live with him instead. Date agrees because he thinks both Renju and Shoko are terrible parents. At some point after his divorce from Shoko, Renju starts dating Futa Amanoma, a.k.a. "Pewter".
1 year before the beginning of the game, Saito regains his memories and concocts a plan to get his original body back and get revenge on Date. He returns to the prototype Psync machine, still in the abandoned chemical plant, and swaps bodies with Shoko Nadami, a rich affiliate of the Kumakura gang and ex-wife of Renju Okiura. Shoko freaks out when she sees that she is in the body of Rohan Kumakura and commits suicide. Saito's plan is to go on a body-swap crusade against Date, swapping with and then eliminating Renju, Iris, So and Boss before finally Date himself.
A short while later, Saito visits Rohan in prison, and Rohan realizes who it is.
9 months before the beginning of the game, Iris is diagnosed with a terminal brain cancer, and given a year to live. It is treatable with the newest technology, but said treatment is prohibitively expensive, and Hitomi cannot hope to pay for it.
1 month before the beginning of the game, Mizuki sees Renju behaving strangely, and takes a taxi to follow him to the cold storage warehouse. After Renju leaves, she opens the trunk of the forklift and sees Manaka's body; she immediately thinks that it's Iris and runs away.
The night before the game begins, Saito switches bodies with Renju. After leaving Shoko's body, he kills Renju who is inside of Shoko's body, swallows her removed eyeball, and displays the body in an amusement park in order to taunt Date. He pretends to be Renju and sends Mizuki a text message saying to come to the park. Meanwhile, Rohan calls So, and informs him that Shoko Nadami knows about the events of 18 years prior.
Mizuki goes to the park with her friend Ota and finds the dead body of her mother. Ota runs off in fear. Mizuki is scared, but pulls an ice pick out of her mother's eye socket, catches a glimpse of Renju on the other side of the merry-go-round, gets startled by a phone ringing (So Sejima calling Shoko's burner phone, which Saito had disposed of inside one of the horses) and hides inside the ride's central column, still clutching the ice pick.
Ota reaches a payphone and leaves an anonymous tip concerning screaming from Bloom Park, attracting the police to the scene.
The Body in Bloom Park
- Bloom Park
Our protagonist, Special Agent Kaname Date, arrives to investigate the crime scene, Bloom Park. With him are his superior, “Boss,” and his partner, Aiba, all of whom work at an organization called ABIS. While Date doesn’t normally inspect crime scenes, Boss asked him to do so here because Date was close to the victim, Shoko Nadami. Date insists that he and Shoko were just acquaintances.
Aiba, short for “AI-ball,” is an artificial intelligence housed in Date’s prosthetic left eye. In addition to her lightning-fast processing power, hacking abilities, and access to anything available online, she can also enhance Date’s eyesight through various vision modes. She directs Date to use her Zoom mode to observe the crime scene while the police finish taking footprints.
Examining the body, Date and Aiba determine that Shoko’s cause of death was blood loss from the multiple stab wounds to the front of her torso, and that she died around 6 hours ago, at about 5:00 pm. The shape of the wounds indicate that the murder weapon was a smooth, pointed object, like an ice pick. The amount of blood coming from her left eye socket indicates that her eye was removed before she was killed.
Boss explains that the Metro Police received an anonymous call around two hours ago. The caller reported screaming coming from Bloom Park, and an officer found the body when they went to look around.
Boss worries aloud that this may be a copycat killer from a case six years ago, but doesn’t elaborate.
The investigation is interrupted by a banging sound coming from inside of the carousel’s central column. Using Aiba’s X-ray mode, Date sees the figure of a young girl with a ponytail sitting there. He rushes to the door, calling to the person inside but not receiving a response. He tries to open the door, but it’s locked from the inside. He shoots the lock open, and the girl looks up at him sadly, a bloody ice pick clutched in her hands. Date recognizes her as “Mizuki.”
- Central Hospital:
- Date’s Somnium:
Aiba explains that her consciousness is directly linked to Date’s through the artificial nerve in Date’s eye socket. She tells Date that she will explain more when he wakes up, since he will forget the whole conversation when he wakes up.
Aiba asks Date if he recognizes where they are, but he is completely unfamiliar with the room.
With that, Aiba surrounds herself with blue sparks and lightning, levitating in the air as she calls out, “Somnium scan, activate!”
She explains to Date that she just scanned his mental world, also known as his Somnium. A Somnium is divided into layers, each with a foundation that must be broken to proceed deeper into the subject’s consciousness. These foundations are called Mental Locks. Because “the truth” is always hidden in the deepest layers of a Somnium, Aiba and Date’s mission is to remove Mental Locks by solving puzzles within the dream worlds that they explore.
Goal: Wake Date Up
Aiba and Date explore the colorless room. Aiba cannot interact with objects until they have been touched by light, which restores both their color and their physical form.
As they progress, they see flashes of unfamiliar people: first a black-haired man in a suit, then a short-haired woman in a white sweater.
Upon completing the puzzles and opening the chained door in the room, Aiba and Date find the man in the man and woman again. The man in the suit is standing, facing away from the woman, who is lying on the ground in a pool of blood coming from her right shoulder. The man starts to turn around, but Date wakes up before getting a look at his face.
- Police Headquarters
Date confirms that he brought Mizuki to the hospital after finding her in the carousel and stayed with her through the night. He recalls their relationship: Mizuki Okiura, a 12 year old 6th grader, lives with Date in his apartment due to “certain circumstances.” He isn’t her legal guardian and they aren’t blood-related, but he is close friends with her father, Renju Okiura. Shoko Nadami, the murder victim from Bloom Park, is Renju’s ex-wife and Mizuki’s mother.
Mizuki is still in the hospital, traumatized and unable to speak. She may have witnessed something critical to the investigation, but the police can’t get any answers while she’s in this state.
Boss tells Date that Renju is currently being interviewed at another police station, though he isn’t currently a suspect in Shoko’s murder. That said, neither Date nor the police were able to call or locate Renju until morning despite multiple phone calls, and his whereabouts the previous night are unknown.
Date asks Boss to elaborate on the case from 6 years ago that she mentioned at the crime scene, but she refuses on the grounds that it’s impossible that the culprit is the same killer as the one in that case, and that knowing more about it would cloud Date’s judgement. All of this only makes Date more curious, but Boss ignores his protests.
In conversation, Boss pokes fun at Date for not actually knowing her name. Date has amnesia: his memory cuts off at a point six years ago, and he remembers nothing about his life before then. He’s been having flashbacks recently, but hasn’t recalled anything of substance.
Regarding the investigation, Boss tells Date that they haven’t made much progress. The police have confirmed that the ice pick Mizuki was holding was the murder weapon, but only Mizuki’s fingerprints were on it. They’ve tracked the call that reported screaming in Bloom Park to a pay phone, so they’ve learned nothing on that front either.
Suddenly, Date’s prosthetic eyeball pops out of its socket. A small, hamster-like body forms around it: the body that Aiba uses in the real world. She wants to tell Date and Boss about her latest finding: using security cam data and facial recognition, she’s identified the anonymous caller as 24-year-old Ota Matsushita. Date and Aiba leave to visit Ota at his address.
- Matsushita Diner: Saturday (Day 2), 10:18 am
Mayumi explains that Ota often hangs out at an entertainment company’s office. Ota is enamored with an idol there and often goes to see her. Mayumi says that she’s a “witch,” using Ota’s attraction and naivety to get him to buy her merchandise. When asked, Mayumi can’t remember the name of the company or the idol, leaving Date and Aiba at a loss. Aiba tries to call Ota’s cell phone, but is unable to connect to it.
Fortunately, a package addressed to Ota is sitting nearby. Date and Aiba use Aiba’s X-ray mode to identify the contents: a figurine with the name “A-set” engraved on the base. Date has a strange feeling upon reading the name, but moves on as Aiba tells him that she’s found information on the figure. A-set is a popular female musician that works for the entertainment company Lemniscate—Renju Okiura’s company. Taken aback, Date and Aiba leave immediately to investigate further.
- Lemniscate: Saturday (Day 2), 11:45 am
Ota has been intensely focused on his laptop for the entire conversation, so Aiba hacks the wi-fi to see what he’s up to. She and Date find that Ota runs four separate To-Witter accounts: three to post insults about A-set, and one (using his real identity) to publicly defend A-set.
Once Date threatens to tell A-set about Ota’s sockpuppet accounts, Ota reluctantly agrees to tell Date about himself and what he knows. A-set, affectionately nicknamed “Tesa” by her fans, is recording a podcast in one of Lemniscate’s studios, and Ota is waiting in the lobby to meet her.
Ota admits to making the anonymous call to the police. He went to Bloom Park the night before to meet his friend, Mizuki Okiura.
Date is shocked to find out that Ota is friends with Mizuki. Ota explains that he met Mizuki through Tesa.
Mizuki called Ota the previous night at around 8:10 pm. She explained that she received an online message telling her to go to Bloom Park’s carousel. She had sounded afraid, so Ota agreed to accompany her as protection.
Ota met Mizuki and they went to the carousel as planned, but stumbled onto Shoko’s corpse there. Terrified, Ota ran away, leaving Mizuki behind. He tried to call the police using his cell phone, but dropped it into a puddle, which is why he used the pay phone.
Date is outraged that Ota abandoned Mizuki, but before they can continue, Tesa arrives in the lobby. Date again has an odd feeling upon meeting her.
Tesa, an 18-year-old high school senior, is very friendly with Ota, who has declared himself her protector both in person and online. She confirms that she befriended Mizuki after meeting her through Renju, then introduced Mizuki to Ota. Tesa is already aware of the incident: Ota told her about what happened, and the story has been circulating online.
Ota didn’t realize that Shoko was related to Mizuki. He heard her name on the news that morning, but didn’t connect them because of their different surnames. He feels incredibly guilty for leaving Mizuki alone with her mother’s body.
After taking their statements, Date prepares to leave Lemniscate and return to Bloom Park to look for more clues. He’s stopped by Tesa, who tells Date that she’s curious about the crime scene and asks to go with him. When Date turns her down, she resorts to blackmail: after giving Ota her phone to record video footage, Tesa suddenly hugs Date and acts out a dramatic plea for him not to leave her. She immediately uploads the footage to the cloud, then tells Date that she’ll delete it if Date lets her come along. Date relents and reluctantly lets Tesa accompany him.
- Date’s Car:
Date asks Tesa about the blackmail video: wouldn’t she also get in trouble if she released it? She admits that she would, but that she isn’t afraid to do so, “because someday real soon… I’m going to die.” After a suddenly solemn moment, she tells Date that she’s joking, but still plans to hold the video over his head.
Tesa presses Date to tell her more about his job. He admits to her (to Aiba’s protests) that he’s a member of a special investigation unit called ABIS, the “Advanced Brain Investigation Squad.” Tesa swears to the Egyptian goddess Isis that she’ll keep this a secret.
Date remembers that he still hasn’t learned Tesa’s real name. When he asks, she tells him that her name is Iris Sagan. Upon hearing this, Date has a sudden, vivid flashback of playing “ratty cake” with a young girl who looks very much like Iris…
- Bloom Park: Saturday (Day 2), 1:12 pm
Iris tells Date that she’s been to Bloom Park before as a child, but hasn’t been able to go since it shut down eight years ago. Aiba recounts the details to Date: The Kabasaki district, where Bloom Park is located, was contaminated by an accident at a nearby chemical plant. The residents were evacuated and the area restricted until it could be decontaminated, but there aren’t enough officers to patrol it heavily. There are several routes leading into the Kabasaki district; some people even live there despite the warnings. Iris, concerned, asks Date whether they’ll be safe investigating there. He reassures her that Bloom Park is almost completely decontaminated and won’t pose any risk to their health.
Iris proposes that Shoko was killed in a ritual murder, possibly by a cult. Date asks her for her reasoning, and she explains that she linked together a number of occult and mythological symbols, beginning with “Horse” sounding like “Horus” and ending with the Eye of Providence (connected to Freemasons and the Illuminati).
Date, skeptical of Iris’s line of thinking, accidentally mentions Aiba while explaining that the police haven’t found any clues. He tells Iris that Aiba is his partner, but doesn’t explain anything more.
Iris tells Date that she heard about the ice pick from Ota. The fact that it was stuck into Shoko’s left eye (the “Eye of Providence”) must have some kind of meaning! Date stops her; he and Aiba didn’t know that the pick had been in Shoko’s eye. He makes a mental note to investigate how Mizuki ended up holding it by the time he found her.
Date takes another look at the scene himself, and realizes that the horse Shoko had been tied to has no blood on it. He and Aiba conclude that her body must have been moved there after Shoko was killed somewhere else. With that, Date is ready to leave and calls Iris to join him.
Iris turns to follow Date, but suddenly feels ill and collapses. Date is concerned, but she brushes him off, joking about having morning sickness. Date offers her a piggyback ride to the car so that he can take her home.
In the car, Iris is a bit subdued, even as she jokes with Date. She opens the window to cool off, despite the mild temperature, and remarks to Date that she feels like she’s met him before somewhere.
- Sagan Residence: Saturday (Day 2), 3:09 pm
Iris invites Date in for tea, and the woman introduces herself as Iris’s mother, Hitomi Sagan. Date is surprised that Iris’s mother is so young; Hitomi confirms that she was only 19 when Iris was born. Hitomi is single (never married), and works as an elementary school teacher. She grew up in the home that she and Iris live in, although she did consider moving away after… something happened, which Hitomi is reluctant to talk about.
Date asks Hitomi about her relationship with Renju. Hitomi tells himi that she and Renju have known each other for 20 years since they were classmates at Eitoku High School. She doesn’t know Shoko well, though, and only met her twice: once at Renju and Shoko’s wedding, and one other time a few months ago. Date decides not to tell her that he’s investigating Shoko’s murder.
Hitomi visibly favors her left arm. When Date asks if she is left-handed, she explains that she is unable to use it anymore. Looking at Date, she asks if he knows her, since she feels like they’ve met before.
When Date asks why Iris why she’s in such a good mood, she points out a winter iris plant by the window, which just bloomed earlier that day.
Date prepares to head back to headquarters. Iris wants to see him again, and Date tells her that they might see each other in the future through their mutual friendship with Mizuki.
- Date’s Car:
Aiba guesses that the message that summoned Mizuki to Bloom Park was sent by the culprit. If Mizuki trusted it enough to follow the instructions, it must have been from someone she knows, or possibly from someone impersonating someone she knows. Aiba starts hacking the server to find out where the message came from, but warns Date that it will take some time.
Date and Aiba agree that Mizuki couldn’t have carried Shoko’s body or transported her from the murder scene to Bloom Park. Date hadn’t seriously believed that Mizuki was the culprit, but they are glad to have actual evidence of her innocence.
They discuss several mysteries surrounding the case:
- Shoko’s body was openly displayed; there’s no way that the killer was trying to hide their crime. Are they seeking fame? Perhaps they’re trying to send a message?
- Shoko’s left eye is nowhere to be found. They now know that the pick was in her eye socket when Mizuki and Ota found the body. Could Mizuki have taken the eye?
- Aiba confirms that security footage shows no cars near Bloom Park on the day of the murder. How did the culprit move Shoko to the carousel?
- Police Headquarters: Saturday (Day 2), 4:26 pm
Boss tells Date that he knows what he has to do: Psync with Mizuki.In the Psync control room, Pewter reassures Date that Psyncing is completely safe, even for a child. Date asks if Renju has given them permission to do this, but Boss tells him that Renju was released from questioning around noon and has disappeared again. Date is hesitant, but Boss reminds him that he may be able to cure Mizuki’s aphonia, and hopefully find a clue they can use in the investigation as well.
Resolved, Date joins Mizuki in the Psync room, where she is hooked up to the enormous Psync machine. He explains the situation and reassures her, promising to go out to eat with her when they’re done.
Date climbs into his seat on the Psync machine and prepares to start. Pewter reminds him that he has only six minutes to find what he needs: any longer and Date’s consciousness could be absorbed by the subject’s. With that, Pewter starts the machine.
On the road to Ota's, Mizuki's, and the Annihilation route, Date and Aiba piece together one of Mizuki's few happy memories with her family. On the road to Iris's and and the Resolution route, Mizuki's memories of being abused by her mother and of finding Manaka's frozen body are uncovered.
The Body in the Cafe
On the road to Ota and Mizuki's routes, Saito's dream represents the animals he was killing as a child, ending with seeing his father having sex with Manaka. On the road to Annihilation, it represents his memory of performing the Original Cyclops Killings.
The Bodies in the Warehouse
Ota's dream is a self-inflation of his fight with the polar bear in order to protect Iris. On the road to his route, he remembers identifying the kitchen knife as the one he gave to his mother. On the road to Mizuki's route, he remembers that he injured the polar bear's leg.
Ota's Route
Mayumi's dream is a representation of the memories she's trying to retain through her dementia.
Mizuki's Route
Date's dream is a recollection of doing his best to look after Mizuki.
Annihilation Route
Psync subject: Prisoner #89 (actually Rohan Kumakura)
Rohan's dream is his own memory of the shooting at the Sagan household, ending as his own body (with Falco inside it) and Boss burst through the door.
Psync subject: Boss (actually Saito Sejima)
Saito's dream is representative of the New Cyclops Killings and his maze of identities.
The Prophecy
Rather than being about Iris as everyone assumes, So's dream is his memory of watching Saito kill Manaka Iwai.
Iris' dream represents the delusions caused by her brain tumor. On the road to her route, Date gets caught up in her delusional beliefs despite Aiba's protests. On the road to Resolution, the delusions are shown to be based on warped versions of her childhood memories and interests.
Iris's Route
Iris' second dream represents her imminent death from cancer and her hope that there's still a chance she can be cured.
Resolution Route
Hitomi's dream is representative of her memory of searching the woods for Manaka, and eventually unearthing her body.
Date's dream is a flashback to the first few swaps done using the normal Psync machine and the prototype Psync machine, as well as Saito forcing himself back into his original body by forcing the Psync to go beyond six minutes.
