Rin Yamazaki is sound asleep having a strange dream which seems to take place underwater, a mysterious figure shows up and Rin questions if they were there. She then gets woken up in a car by Seitaro and starts a small argument.
Yuka Kikuchi asks Rin if she slept well, which Rin replies that she was until Seitaro rudely woke her up which she calls him 'baldy' for having gray hair. Seitaro asks Yuuta Takahashi, who was driving the car, if they were in the mountains yet but Yuuta tells them that he has no idea where they are, which made Rin kick Yuuta's seat in anger, and comments that she didn't want to go on a trip in his 'lousy ol' GPS-less car'. After Yuuta feeling hurt from the kick and telling Rin to not kick his seat, suddenly the car stops. Rin angrily says that she'll check on the car and gets out. Seeing nothing wrong with it, she questions on why it stopped. After walking forward, an old man is seen asking Rin why she's in the mountains.
After being startled, the old man offers Rin and her friends to stay at his mansion until their car was fixed, which all of them agreed to and followed the old man back to his home. The old man shows everyone into their rooms. After showing Rin her room and telling her that he'll be preparing dinner for them, assuming that they were hungry. Rin will go outside and comment on the huge swamp and tries to read the stone monument near it, the old man will ask her if she was curious and tells her the legend of the mermaid swamp. After telling Rin the legend, the old man tells her that dinner was ready, to Rin's relief.
Rin finishes her dinner first and tells them to clean up after her, which Seitaro tells her to do it herself, and Rin replying to him to 'work on his names while he's at it'. After taking a bath, Rin will go to sleep and dream about being underwater again and sees the figure again and tries to tell it to stop staring or else she'll kill it.
Rin wakes up after having the dream. After talking to Seitaro and Yuuta in the living room about the car still not looking any better, the old man will tell them that he had to go to a memorial service for a few days and leaves the mansion to them, and tells them not to go walking in the mountains as well. After the old man leaves, Rin will get into the bath and starts worrying about Yuka because she started to feel cold. Rin starts to become drowsy and falls asleep in the bath. She wakes up to find the bath water all filthy and disgusting. She freaks out and Seitaro comes in and asks what's wrong and sees her naked, which she furiously hits him for barging in unexpectedly. After talking about the bath water which appeared to be swamp water according to Seitaro, Rin checks up on Yuka, she then goes back into her room and charges her cellphone and goes to bed. After falling asleep, Rin dreams about the mansion filled with water. After walking out of her room, she sees a dark figure going towards Yuka's room. Rin follows it and sees it near Yuka. After waking up, she goes to check on Yuka and finds her body all swollen and screams. Seitaro hears her and sees Yuka. He tells Rin to call an ambulance using the phone in the living room. After using the living room phone which wasn't working because the power is out, Rin went to get her cellphone only to find it that it was out of battery. After talking to Yuuta about Yuka's unexplainable swelling, Seitaro, who went out on the mountains, tells them to help search for a map to get out and get Yuka help. Looking around, Rin questions about whether to light the fire in Yuka's room after hearing her mumble that she was cold and Yuuta acting weird. After searching, Seitaro will give her a lighter and a lantern to give to Yuuta, which Rin heard him jerking in his room and leaves the stuff near his door. Worrying too much about Yuka to sleep, Rin searches again and was able to find some firewood in the storeroom. If she lights the fire in Yuka's room, Rin will comfort her and tells her she'll be OK and they'll get her to a hospital soon.If she doesn't light the fire, Rin can still continue her search for the map. Rin then goes to the old mansion and sees somebody in the swamp but soon find that there was nobody there. Yuuta comes up from behind, startling Rin. He asks her why she was up and tells her to go back to her room and rest which she refused at first until he stomps forward and tells her to go back again, which this time she agrees and goes back. Rin falls asleep and sees two little kids talking about going to their grandma to talk about the 'fishy fish'. Afterwards, they are seen talking to their grandmother about the mermaid legend. The grandma tells them that the mermaids are angry at them when asked if the 'fishy fish' are mad. She then says that "But outsiders... Outsiders will be cursed by her.", which is why "No outsiders may come to this house."
Rin will wake up and go check on Yuka and sees Yuuta coming out of her room. After checking on Yuka, Rin will wake Seitaro up calling him "Early-baldy" and tells him to make breakfast for her. After waking him up, they'll be sitting at the dining room table eating snacks and talk about getting Yuka to a hospital. After having finished eating, Rin will go to the old mansion again. After standing on a table to get a spring from the top of a high bookshelf, Rin falls off and becomes unconscious. She then hears someone telling her to get up and finds Yuka back to normal, and Rin yells at her for making her worried then saying that they should forget about the map because Yuka might be hungry. Yuka stops Rin and tells her a secret, saying "I'm a hostage here.". Rin will see Yuka shaking and Yuka telling Rin to help her, she backs up to see Yuka leaking swamp water and coming towards her, yelling that she's cold and to take her back. Rin then finds Seitaro waking her up and telling her she can't be relied on for anything. Rin searches the old mansion more and finds a golf club in one of the rooms. Rin was about to leave the room when an old phone rings and Rin answers it. The phone call will be from Yuka and she'll either say "I'm sorry" if Rin lit the fire or "Help me" if she didn't. After Rin goes back to the main mansion, if she lit the fire she'll find Yuka dead and screams, which Seitaro will come in and tells Rin to get out of the room. Rin will be sitting outside her room with her knees up to her face, asking Seitaro what happened to Yuka to which he replies that it was no good and that lighting the fire sped up the process of her dying because "even her organs couldn't take it". Blaming herself for lighting the fire and Seitaro saying that it wasn't her fault, Yuuta will come out of his room and ask what happened which Rin and Seitaro didn't reply. Yuuta will check on Yuka and run to the kitchen and stab himself. Seitaro will lay clothes over his body and tells Rin not to go in there. If Rin goes back and didn't light the fire, she'll find Yuuta next to Yuka's bedside and yells at him to get away from her. After Yuuta compliments on Yuka and why Rin can't be like her, he brings out a knife and Rin realizes that the Ophelia paintings in the mansion caused Yuuta's insanity. Rin will go around the mansion with the golf club and smash all of the Ophelia paintings. Once she smashed the last painting in the old mansion, she finds Yuuta collapsed on the ground and Seitaro coming in and finds Yuuta on the ground as well. Rin will explain what happened to Yuuta which Seitaro disagreed with, Yuuta will wake up and Rin will kick him yelling at him for chasing her around with a knife. Rin will talk to Seitaro about her going to get help which led Seitaro to argue with her stating that she'll get lost in the heavy fog and that he'll have to go after her. Not listening to Seitaro, Rin will go up into the mountains and collapse from exhaustion and sees Yuka again. Rin will either say that she's dead or she's at the mansion. After hearing Yuka wanting Rin to take her back, Rin states that she's the one who wants to be taken back. Seitaro will go find her and sees her crying. After asking if she was OK, she faints and she's carried back to the mansion. Rin will be in bed while Seitaro comments on her reckless decision. She'll talk about the mermaid's curse and that it was the cause of Yuka's illness and Yuuta's insanity, but Seitaro tells her that she's thinking differently from fatigue and there was no such thing. He'll state he's at their ropes end and he will suddenly have a headache which worries Rin. Still thinking it's the curse, Rin remembers the name "mermaid swamp" and decides to jump in the swamp to find any clues. After swimming towards the bottom, Rin finds a hairpin at the bottom.
A black shadow then starts swallowing her and she couldn't breathe. Seitaro jumps in the swamp and argues with Rin, telling her to do something productive like finding a map. Seitaro will have another headache and Rin apologizes saying that it was her fault that he's not feeling well. After getting dark again, Rin will go back to her room but states that she's too afraid to sleep and goes back to the old mansion. She finds a hidden room in the fireplace and finds a somewhere map, but she senses someone in the room and tries to escape. But she accidentally drops the lantern, suddenly sees the mermaid and then promptly faints at the sight.
She wakes up and goes back to the upper room to find out she slept until morning. Rin will see Seitaro waiting for her and he asks where she went and explained she found a secret room with the map and asks if he could look at it. But instead Seitaro goes up to her and hugs her, which makes Rin blush and telling him to let go. Asking him what's wrong, Seitaro says that he's scared and about what happened to Yuka and Yuuta and wanting Rin to stay by his side.
Rin can accept and hug Seitaro back, apologizing for making him worried. She'll say that they should go back to the main building to which he agreed, but when Rin started walking back, Seitaro knocked her out cold. Rin will see she's underwater again, thinking that it was only a dream, but finds out that it wasn't a dream and that she couldn't breathe. Crying out for help and for Seitaro, he is drowning her in the bathtub at the main mansion. This will result in a game over.
Rin can refuse and she'll push Seitaro away and yell at him that he's gone crazy too. Seitaro will bring out an axe and tries to get Rin to come with him but Rin shoves him away again and runs. Rin will keep running until she's in the first west side door on the second floor and Seitaro will corner her and tells her that she can run, but she can't hide. Rin could choose to dodge Seitaro's attack which he misses and falls through the floor. She'll go back downstairs to the room beneath to find Seitaro with an axe in his stomach, all bloody. She'll panic and try to think of a way to stop the blood but Seitaro will tell her to take the axe out. After doing so, Seitaro will get up and tells Rin to stay away from him and walks out. Rin will follow him in front of the swamp seeing him about to fall into the swamp. She'll cry out that she didn't want to be alone and she needed him but he told her to escape, either on her own or with Yuka and Yuuta if the fire wasn't lit. He tells her that he wanted her to live and finally falls into the swamp. Rin will go after him and try to reach for him but fails and goes back up to the surface and cries. Yuuta will come out and ask Rin why there was blood on the ground which she answers that Seitaro took an axe to the stomach and fell into the swamp. Yuuta will panic and tell Rin that he'll fix the car and they'll get out soon. She'll stop crying and finds Seitaro's wristwatch at the shore and looks at the map. She'll go to the study and read a book about telling directions and finds out that there was something across the swamp. She uses a motor boat and gets her way across and finds a grate which led to an underground cave, after walking in she'll state that she's freezing cold and keeps walking until she finds a door. Rin can also not dodge Seitaro's attack and kicks him in the stomach instead, making him drop the axe and fall onto the floor. She'll kick him a couple of times out of anger yelling at him to never chase her with an axe again and calling him a creep, but then she'll realize that he was unconscious and waits for him to wake up. After he groaned about his stomach hurting, she'll kick him again and they start arguing about what happened. Still believing in the curse, Seitaro still disagrees and Rin almost yelled out that he likes her until he yells at her to stop. He admitted that it's the curse and said he'll help her. Rin will ask Seitaro in the study a few things and borrow his wristwatch and he'll tell her how to check directions. She'll find out that there's something across the swamp and use the motor boat, Seitaro will come along as well. They'll find a grate covered with grass and walk in which leads to an underground tunnel. On the way to the room, Rin will stop and Seitaro will ask Rin what was wrong and she'll reply that she's cold, which led to Seitaro giving her his jacket for warmth. They'll keep walking until Seitaro finds a door and tells Rin that they should go in.
