Warning: There may be some spoilers below!
Yoo Jin Woo, CEO of tech company J-One, travels to Granada to meet up with a mysterious teenage boy, Jung Se Joo, who offers to sell him an AR (augmented reality) video game. He stays at a run-down Korean hostel called Hostel Bonita and acts like a jerk toward the owner, Jung Hee Joo, who dislikes him because of it. Jin Woo tries out Se Joo’s game and is astounded by the detail in the AR game. But the boy also has other interested buyers, including Jin Woo’s archnemesis, Cha Hyeong Seok. Meanwhile, Se Joo disappears on his train back to Granada. Jin Woo finds out that because Se Joo is a minor, his guardian is the one who has the power to sell his game. Unfortunately, his guardian also happens to be Hee Joo.
Jin Woo tries to get on Hee Joo’s good side, but she’s not having any of it. He finds out that Se Joo is supposed to arrive at the train station that morning and goes to meet him, but he doesn’t show up. Instead, Jin Woo runs into his ex-wife, Su Jin (who cheated on him with his former best friend Hyeong Seok and is now pregnant) and finds out that Hyeong Seok is in Granada as well. Jin Woo enters the game and sees that a new user has logged in — he goes to find the new user and it turns out to be Hyeong Seok. Hyeong Seok tries to taunt Jin Woo into dueling, but his level is higher than Jin Woo’s, so Jin Woo backs down. Jin Woo has his assistant reveal his identity to Hee Joo, who immediately treats him differently upon finding out he’s the rich CEO. He approaches her with a business proposal. Flash-forward a year and Jin Woo is on a train to Granada in much different circumstances — his clothes look ragged and his leg is injured. The weather suddenly turns stormy and he begins fighting with masked bandits on the train.
Jin Woo offers Hee Joo 10 billion won in order to buy her family business after finding out that Se Joo applied for a patent for the video game under the family business’s name. He learns a bit about Hee Joo’s past and discovers that her family moved to Spain in order for to pursue music, specifically classical guitar, but they soon fell on hard times after her mother and father both passed away. Hee Joo is apprehensive about the contract at first, but the money is tempting and she decides to sign. Jin Woo discovers that Se Joo made his sister into a classical guitar-playing NPC named Emma and becomes enthralled with her. He encourages the real Hee Joo to try playing guitar again. Jin Woo levels up so that he’s the same level as Hyeong Seok, then calls him out at night so they can duel. He wins. Jin Woo flies back to Barcelona. Hee Joo seems a bit disappointed to find out that he probably won’t return to Granada again. The next morning, Jin Woo gets a call from his assistant — Hyeong Seok was found dead in the park where Jin Woo had left him.
Jin Woo returns to Granada after finding out about Hyeong Seok’s death. He asks Hee Joo to help take care of Su Jin after she faints upon hearing about Hyeong Seok’s death. The police ask Jin Woo to go in for questioning after they find out he was the last person to speak to Hyeong Seok. Jin Woo returns to the park where he met Hyeong Seok and enters the game. Hyeong Seok’s bloody corpse is still there. It disappears, then reappears as a live NPC in the game. Jin Woo fights him off. He gets a call from the train station asking him to claim a lost bag — it turns out to be Se Joo’s, but has nothing useful in it. He goes to Hee Joo’s house to do some more investigation and finds that Se Joo contacted a man named Marco in Barcelona. Jin Woo gets a call from Hyeong Seok’s father. NPC Hyeong Seok reappears at the Hostel Bonita and attacks Jin Woo in his room. They fight. Hee Joo goes to check on Jin Woo. She’s on her way up the stairs when Jin Woo falls through the center of the staircase down to the first floor.
Hee Joo gets a call from Go Yu Ra, Jin Woo’s wife, who demands to speak to him. She goes upstairs to wake him up when Jin Woo falls from the top floor. He eventually wakes up in the hospital with multiple fractures and a leg injury he will never recover from. Jin Woo finds out that he was not in the game when NPC Hyeong Seok attacked him — at the very least, he didn’t have his lenses on. Professor Cha, Hyeong Seok’s father, finds out that Jin Woo may have been involved in Hyeong Seok’s death, and the authorities suspect that Jin Woo tried to commit suicide. We learn that Professor Cha had something to do with the falling out between Hyeong Seok and Jin Woo, and that he disowned his son. Hyeong Seok appears again at the hospital. Jin Woo tries to run away, but just looks crazy to everyone else. He admits to Hee Joo that he thinks he’s going crazy and realizes this is why Se Joo disappeared.
Hee Joo returns to the hostel after Yu Ra blocks her from seeing Jin Woo. But later that night, Jin Woo appears, asking for his lenses. She won’t let him leave alone, so Hee Joo goes with him. Jin Woo breaks into an antiques shop that doubles as a weapons store in-game. Hyeong Seok appears again and they fight, with Jin Woo killing him yet again. Hee Joo finds Jin Woo in one of the antique store’s storage rooms. Professor Cha decides to not move forward with the investigation into Hyeong Seok’s death. Jin Woo spends the next few days in a drugged up haze in order to avoid facing Hyeong Seok again. Hee Joo stays with him. When her birthday rolls around, he tells her to go out and have fun and tells his secretary to send her a bouquet of roses. But he freaks out when all alone and barricades himself in his shower. His secretary finds him there and Jin Woo decides that they should leave Granada that night. Hee Joo thinks of Jin Woo during her birthday celebration and calls to check on him. When she finds out that he’s leaving, she rushes to the train station, but just misses him. He watches her sprint onto the platform as the train pulls away.
Grandma starts wondering where Se Joo is but doesn’t do anything about it. Hee Joo tells her that she sent him an email and he read it. Later, she gets an email from “Master” and smiles. One year later, testers are trying out the AR game in Seoul. Meanwhile, Jin Woo’s employees are hard at work in the game studio showing off all the different versions of the game they’re developing for different cities. Park goes to Hyeong Seok’s memorial service, where Jin Woo appears halfway through, shocking everyone. He tells Professor Cha and Park that he’s better now and doesn’t hallucinate anymore, but later in the service sees Hyeong Seok appear again and kills him with one shot from a handgun. It turns out that Jin Woo hasn’t been in America and instead has spent the past three months gaming day and night in order to level up and feel safer in his encounters with Hyeong Seok. Meanwhile, Go Yu Ra has been pretending to be at his bedside caring for him for the past year and is furious when his company releases a statement that he was not in the States. Jin Woo dulls his pain through a combination of alcohol and medication and doesn’t seem to care when Park questions him on his whereabouts. He spots Hyeong Seok again, and this time his secretary bursts into his office and slays the NPC. Hyeong Seok has had Jeong Hun send Hee Joo emails, pretending to be Se Joo, while they search for Se Joo. But Hee Joo figures out that Jin Woo has been lying to him this whole time, and confronts him when he finally decides to show up at her workshop/house.