Steins;Gate 0

=== Setting ===

Steins;Gate set in Akihabara, Tokyo. It follows several characters, including the university student Okabe "Okarin" Rintaro, who, together with his friends Shiina "Mayushii" Mayuri, the neuroscientist Makise Kurisu and the hacker Hashida "Daru" Itaru, accidentally have discovered time travel through the use of a microwave oven and a phone – the PhoneWave – which they used to send text messages and digitized memory data back in time. The latter, referred to as "time leaping", essentially creates a copy of memories from a person's mind and is intended to send this "knowledge/digitalized data" back in time and into the same user's mind, at an earlier point. This has drawn the attention of SERN (clearly referring to the real CERN), an organization that secretly researches time travel, who sent their team of "Rounders" to confiscate the time machine, including the journalist Kiryu Moeka, killing Mayuri in the process.
Within the setting, time does not consist only of alternative time lines. Each time line is different, and the "unit measure" that defines this difference is called divergence. The detector to measure this is called a divergence meter which was set to 0% when it was created. When it travels into another time line, it measures the divergence (difference) of this new time line from its original. Time lines that are minor variations on a greater "reality/history course" are minor branches on what is called a world line, basically a whole series of connected time lines. This means certain events are fixed points, destined to happen in all branch time lines, unless divergence is enough to lead to another world line. On a greater scale is the idea of an attractor field, a larger scale version of a world line. One important fixed point in the story is Mayuri's death in 2010, which must happen in any time line belonging to the Alpha attractor field. To prevent Mayuri's death, Okabe needs to make a significant enough change to cause a shift from the Alpha attractor field to Beta, where instead Kurisu destined to die in 2010 and in which time travel is not discovered. Between the Alpha and Beta attractor fields lies the Steins Gate world line, unaffected by their convergence points. Some people can retain memories from the previous time lines after a shift, Okabe in particular possessing a very strong effect. He calls it Reading Steiner.
Steins;Gate 0 is a prequel to Steins;Gate. Among other characters are Daru's future daughter, the time traveler Amane Suzuha; Mayuri's future adoptive daughter Shiina Kagari; Kurisu's co-workers Hiyajo Maho, Alexis Leskinen and Judy Reyes from Viktor Chondria University; Okabe's friends Urushibara Luka and Akiha "Faris NyanNyan" Rumiho; and Mayuri's friends Amane Yuki, Nakase "Fubuki" Katsumi and Kurushima Kaede.

Plot

The game begins with a scene similar to Steins;Gate's ending, where Suzuha traveled to August 21, 2010, using her time machine to get Okabe to prevent a time-travel arms race leading to World War III; to do this, he needs to stop Kurisu's father, Doctor Nakabachi, from killing Kurisu and bringing her time travel theories to Russia. Suzuha brings Okabe to the moment of the murder, July 28, 2010, but he accidentally kills Kurisu himself, seemingly due to world-line convergence. Upon returning to the present, Okabe refuses Suzuha's requests to try again, and develops post-traumatic stress disorder. Nearly five months later, Okabe attends Kurisu's coworkers Maho and Leskinen's presentation of their Amadeus system, which uses digitized memories as artificial intelligence avatars; one avatar has been made based on Kurisu. Speaking with them, Okabe becomes a tester for Amadeus, allowing him to communicate with Amadeus Kurisu through his phone. While trying to change Okabe's mind, Suzuha looks for Kagari, who got separated when traveling to 1998. Maho, who has Kurisu's hard drive, wants to access her theories, hoping to be able to save Kurisu; Russia, other countries, and groups including SERN are also after the theories, and the world line shifts as they monitor activities surrounding Kurisu's theories and memories.
The story splits into two major branches: In the one leading to the ending "Promised Rinascimento", Okabe rebuilds the PhoneWave to help an amnesiac woman identified as Kagari, who has had Kurisu's memories implanted by someone, which gradually overwrites her personality; this inadvertently transports him to 2036 during World War III. Although Okabe had been predetermined to die sometime in 2025, Daru circumvented it by implanting 2011 memory data, as Okabe's body was still alive. Inspired by this and determined to avert this future, Okabe returns to 2010, destroys the hard drive, erases Amadeus, and sends a D-RINE message to the past of this branch, telling himself to "deceive the world and tie the possibilities". The resulting world line is not shown.
The first major branch has two side branches: In "Recursive Mother Goose", Okabe and Mayuri trace the origin of Kagari's only memory, a song they learn Okabe originally sung as a child after Mayuri's grandmother's death. After nearly being hit by a truck, Kagari regains memories, including having escaped a facility she was held at in 2005. Mayuri intends to time travel with Suzuha to reach Steins Gate, but Kagari takes Mayuri's place as she fears not meeting her in Steins Gate.
In "Twin Automata", Maho hides in Faris's apartment after a kidnapping attempt, and befriends Moeka. Noticing that somebody is trying to steal Amadeus, Leskinen and Maho go to lock down the data, but they are ambushed by Reyes, who shoots Leskinen and tries to make Maho give her the Amadeus access codes. Maho decides to erase Amadeus rather than give it up, and she is saved when Moeka shoots Reyes. While Amadeus Kurisu is being deleted, a Kurisu apparently from another worldline takes her perspective. She calls for help, and tells Maho that Steins Gate is real, and tells Maho her laptop password is related to Piano Sonata No. 10, a piece by Mozart that had been the theme for this ending branch. Returning home, Maho figures out Kurisu's password from this hint and finds her research.
In the second major branch, leading to "Vega and Altair", Okabe stops testing Amadeus. If the "Promised Rinascimento" ending was completed first, a D-RINE message will appear early into the branch with its cryptic message: "deceive the world and tie the possibilities", but its origin is unknown. He and Maho are followed by groups who want Kurisu's theories, and Kurisu's hard drive is destroyed in a skirmish. Daru and Maho secretly rebuild the PhoneWave in an effort to save Kurisu, but is unable to complete the test due to missing components; Okabe tells them that saving Kurisu means sacrificing Mayuri, which Mayuri overhears. She decides to travel back in time with Suzuha to convince Okabe in 2010 to save Kurisu after failing in his prior attempt. However, Leskinen appears, revealing that he works for the intelligence agency Strategic Focus and intends to steal the time machine. Mayuri and Suzuha attempt to flee in the time machine, but it is hit by a rocket and destroyed. In the aftermath, Okabe discloses the missing components to Daru and Maho to complete the PhoneWave, thus allowing him to time leap to an earlier part of the day, and works to ensure Mayuri and Suzuha's successful time travel. Surrounded by Leskinen's men, he declares that he will find a way to Steins Gate.
The second major branch has the side branch "Gehenna's Stigma", where Maho is caught eavesdropping on Reyes and Leskinen, hearing that Leskinen thinks Okabe hides information. Leskinen uses Maho: She meets Okabe, who tells her about time travel provided that she does not try to save Kurisu. After he reveals where Suzuha's time machine is, Maho suffocates him; when waking up, he is tortured for information by Leskinen. Wracked with guilt for betraying Okabe, Maho makes an unsuccessful attempt to save Kurisu in her own way. World War III begins, Daru flees Akihabara to develop the time machine, and Okabe gives up on reaching Steins Gate.
The final ending, "Milky-way Crossing", only appears after completing the "Promised Rinascimento" and "Vega and Altair" endings. It is set in 2025 on a world line similar to, but not the same as, the one from "Vega and Altair". Okabe sends a D-Mail video to his 2010 counterpart, instructing himself how to save Kurisu and cause the world line divergence into Steins Gate, thus setting the events that would play out as the True End route in the previous game. He then leaves in the time machine to find Suzuha and Mayuri, and bring them safely back to 2025.