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.hack//Quantum (クワンタム) is an OVA series, part of the 3rd Era of the .hack saga, and the first in the series to be licensed by Funimation. The series consists of three episodes and was presented by Bandai Visual company. Animation was produced by Kinema Citrus (Tokyo Magnitude 8.0) with director Masaki Tachibana (Tokyo Magnitude 8.0, Noir) taking charge of the animation. .hack//writer Tatsuya Hamazaki wrote the OVAs and video game and anime composer Ko Otani (Shadow of the Colossus, Blade of the Immortal) composed the music for the series. CC2 confirmed that Quantum is not related to the CGI movie tie-in to .hack//Link.
Episode 1: Walking Party
In 2022, Asumi, Eri, and their friend Iori all play The World R:X together as Sakuya, Mary, and Tobias respectively. On one occasion, they happen to be in the dungeon containing The One Sin event while Shamrock's guild was trying to defeat it. While trying to reach a good spot to watch from, Sakuya accidentally triggers a trap that instantly kills many of the guild members.
The next day, Iori finds that Shamrock's guild placed a bounty for Iori's character Tobias, along with Sakuya and Mary. Sakuya decides to log in anyways, hiding out by the riverbank in Mac Anu. There, she meets Hermit. While they are talking, Iyoten and Asta capture them with cheat items. However, Mary and Tobias show up, figuring that Sakuya would log in anyways. Hermit uses his powers to erase the cheat items and repair the damage to the surrounding area that they caused while Sakuya, Mary, and Tobias fight with Iyoten and Asta. As the fight turns into a chase, Shamrock and some of her guild members try to stop Iyoten and Asta. It is then revealed that Iyoten and Asta were the ones who posted the bounty, not Shamrock. Soon after, Iyoten PKs hermit by pushing him into an abyss and all of the characters nearby are transported to an unusual area where a strange AI appears to be corrupting PCs and putting them into cages hanging from the branches of an enormous dead tree. Unsure of what's going on, Tobias tells Sakuya and Mary to log off before logging off himself. Sakuya tries to log off, but the AI interrupts the process. Mary attacks the AI, who then corrupts her. Sakuya reaches out to help Mary, but she gets knocked out.
Episode 2: Wired Prisoner
Hermit wakes up in the strange area to find that everyone was gone or had been corrupted by the AI. He starts looking at the items the players left behind trying to find his list until he gives up and hires Geek to make a new one. Hermit starts using it to identify the corrupted players.
Meanwhile, Sakuya's player, Asumi, wakes up to find that she and Mary's player, Eri, are in the hospital. Asumi was only unconscious for a few hours and the only long-term effect is a tingling in her right hand, the same hand which she used as Sakuya to touch Mary. Asumi is told that she and Eri had just been playing for too long and that Eri would soon recover, but as time goes by, Asumi and Iori become suspicious.
Sakuya finds Hermit's original list in her items, not knowing when she picked it up. Together with Iori, she looks into it. They are then contacted by a mysterious woman who tells them where to meet up with her in the real world. This person turns out to be the player behind Shamrock, Reiko Saeki.
Meanwhile, Hermit meets with Geek and expresses his desire to free the corrupted players in the strange area. Geek tells Hermit that the players' minds were hooked up in a kind of quantum computer and that freeing them would be difficult.
Later, Sakuya, Tobias, and Shamrock meet up again online, and Shamrock explains the list to Sakuya and Tobias. She says that the list documents the haplotypes of players in The World and compares them with the haplotype of a young American boy who died seven years previously. While they are talking, Hermit comes up and spies on them, noticing that they have his list. When he hears that the boy whose haplotype was being compared is dead, he is shocked and teleports away. He then goes to the bar where Geek is waiting to speak with him and starts trashing the place in blind anger, yelling, "All of this and for nothing! Seven years just wasted!" Meanwhile, Tobias questions Shamrock about a hacker's ability to access personal information. Shamrock proves that the list can hack into players' information by pulling up Sakuya. When she does, however, the list shows that Sakuya is a compatable donor. Just then, the Mac Anu is overrun by data bugs and the dead tree with the caged players hanging from its branches starts growing into the town. A corrupted form of Eri appears in front of Sakuya and attacks her. Shamrock uses a form of data drain to fight off the corrupted Eri, but it's ineffective. Shamrock then kills all of the players in the area with an area attack to force them out of the game before the data bugs can kill them.
Episode 3: The Worldend Pallbearer
Shamrock analyzes Sakuya's right hand and determines that Sakuya/Asumi is a partial lost one, since her hand has been absorbed by the system (leading to the real-life tingling and the fact that she can feel things in-game that are done to that hand). Sakuya also tells Shamrock that on the night that Mac Anu was invaded, Eri improved for a little while in real life before going back deeply into her coma, proving that the corrupted Eri they saw in game was connected to the real Eri and that the data drain temporarily helped her.
Shamrock expresses a suspicion that the corrupted data might target a large-scale event next, but she doesn't have enough evidence to shut down the event. She warns Sakuya to not log in again for a while, since she's the target. Tobias then goes to the event to investigate a strange forum post Asumi told her about. While there, Tobias sees that the players actually feel the pain of the attacks, and then some massive data bugs show up. As Asumi scrolls through forum posts, she sees a post about the data bugs and tries to call Iori. Iori doesn't respond as she has been pulled into the game like everyone else in that event. As Tobias, Iori leads the other players in defending themselves against the data bugs. Just then, Sakuya logs on and comes to help. While defending herself, Sakuya accidentally deletes the data bug with her "lost" hand, allowing the others a chance to escape. As they try to escape, they find themselves being chased by an enormous statue that has been animated by the corrupted data.
Meanwhile, Hermit goes a little crazy and deletes a player and destroys an airship.
Shortly afterwards, Shamrock detains Geek and questions him about the new list he compiled and his employer, Hermit. Geek refuses to answer the questions, but Shamrock hits him to remind him of the reality of the situation now that they're trapped in-game and can feel pain. Shamrock shows Geek some footage of the event and all of the data bugs and giant statue. She comments on how the person responsible is not holding back, and how they couldn't either. She hacks into an enormous weapon called the crest gun.
Hermit is then shown with the corrupted Eri. He tells her that Sakuya is waiting for them.
The players running from the event make it back to town, though they aren't safe as the data bugs followed them. Sakuya comes into the room where Shamrock is. Shamrock chastises Sakuya for logging in, but then has Sakuya use the power in her hand to finish hacking the crest gun. Shamrock instructs her guild members to open the gates for the chaos gate section of town and to bring all the players in there for safety.
Hermit is shown again, alone, commenting on how lame it was that the event ended early. He again expresses his desire to see Sakuya again. He sends Sakuya a message telling her that Eri is in Megin Fi. Sakuya receives the message and tells Tobias that she's going to find Eri. Shamrock tells her guild members to stop Sakuya from going alone. She ties Sakuya up and sends Tobias out to Megin Fi without her. Tobias sees "blood vessles" in Megin Fi and follows them downward, looking for the "heart" controlling the problem at hand. Tobias makes it, but he is tied up by Eri and Hermit tells him that he's not allowed to destroy the heart until Sakuya comes. Meanwhile, Sakuya uses her power to break the ropes that tied her up and then she starts rushing toward the chaos gate.
- Various characters from the .hack franchise can be seen during the 3 OVAs. Some of them are not necessarily the characters themselves, but rather players using similar models. Some of those include Atoli, Aina. Alkaid, Piros the 3rd, Natsume, Grein, Silabus, Gaspard, Kuhn, Orca, Wiseman. The real IYOTEN and Asta appear. Phantoms of the actual Ovan, Shino, Tsukasa, and Aura are seen. Sakuya, Mary and Tobias are modified models of Kite, BlackRose and Balmung, respectively.
- Shamrock's Guild, Yata Mirror, is a reference to Yata's chamber in the Serpent of Lore.
- In the final episode of Quantum, when players are seen trying to keep the infected bugs out by pinning themselves onto the gate, Orca, or a player similar to Orca, can be seen in the middle of the group. Silver Knight also appears, helping the remaining players to escape to the Chaos Gates (although it's not confirmed if that's him).
- Interestingly, Sakuya's infected hand is her right arm, which matches Kite/Shugo Twilight Bracelet's on their right arms.
- The episode titles Walking Party, Wired Prisoner, and the Worldend Pallbearer can be shortened to WP. This may be a reference to wave-particle duality, which is a "central concept of quantum mechanics".
- Players in this series are able to create characters who's model types are named after characters from previous games and anime in the .hack series. The main characters of this series are Kite type, Black Rose type, and Balmung type.