On July 20, 1969, astronaut Jim Lovell hosts a party where guests watch Neil Armstrong 's televised first steps on the Moon from Apollo 11. Lovell, who orbited the Moon on Apollo 8, tells his wife Marilyn that he will return to the Moon to walk on its surface. Three months later, as Lovell is conducting a VIP tour of NASA's Vehicle Assembly Building, his boss Deke Slayton informs him that his crew will fly Apollo 13 instead of 14, swapping flights with Alan Shepard 's crew. Lovell, Ken Mattingly, and Fred Haise train for their mission. Days before launch in April 1970, Mattingly is exposed to German measles, and the flight surgeon demands his replacement with Mattingly's backup, Jack Swigert. Lovell resists breaking up his team, but relents when Slayton threatens to bump his crew to a later mission. As the launch date approaches, Marilyn has a nightmare about her husband dying in space, and tells Lovell she will not go to Kennedy Space Center to see him off for an unprecedented fourth launch. She later changes her mind and surprises him. On launch day, Flight Director Gene Kranz in Houston's Mission Control Center gives the go for launch. As the Saturn V rocket climbs through the atmosphere, a second stage engine cuts off prematurely, but the craft reaches its Earth parking orbit. After the third stage fires again to send Apollo 13 to the Moon, Swigert performs the maneuver to turn the Command Module Odyssey around to dock with the Lunar Module Aquarius and pull it away from the spent rocket. Three days into the mission, by order of Mission Control, Swigert turns on the liquid oxygen stirring fans. An electrical short causes a tank to explode, emptying its contents into space and sending the craft tumbling. The other tank is soon found to be leaking. Consumables manager Sy Liebergot convinces Kranz that shutting off two of Odyssey ' s three fuel cells offers the best chance to stop the leak, but this does not work. With only one fuel cell, mission rules dictate the Moon landing be aborted. Lovell and Haise power up Aquarius to use as a "lifeboat", while Swigert shuts down Odyssey to save its battery power for the return to Earth. Kranz charges his team with bringing the astronauts home, declaring " failure is not an option ". Consumables manager John Aaron recruits Mattingly to help him improvise a procedure to restart Odyssey for the landing on Earth. As the crew watches the Moon pass beneath them, Lovell laments his lost dream of walking on its surface, then turns his crew's attention to the business of getting home. With Aquarius running on minimal electrical power and rationed water supply, the crew suffers from freezing conditions, and Haise develops a urinary tract infection. Swigert suspects Mission Control is concealing the fact they are doomed; Haise angrily blames Swigert's inexperience for the accident; but Lovell quashes the argument. As Aquarius ' s carbon dioxide filters run out, concentration of the gas approaches a dangerous level. Ground control improvises a "Rube Goldberg" device to make the Command Module 's incompatible filter cartridges work in the Lunar Module. With Aquarius ' s navigation systems shut down, the crew makes a vital course correction manually by steering the Lunar Module and controlling its engine. Mattingly and Aaron struggle to find a way to power up the Command Module systems without drawing too much power, and finally read the procedure to Swigert, who restarts Odyssey by drawing the extra power from Aquarius. When the crew jettisons the Service Module, they are surprised by the extent of the damage, raising the possibility that the ablative heat shield was compromised. As they release Aquarius and re-enter the Earth's atmosphere, no one is sure that Odyssey ' s heat shield is intact. The tense period of radio silence due to ionization blackout is longer than normal, but the astronauts report all is well, and the world watches Odyssey splash down and celebrates their return. As helicopters bring the crew aboard the USS Iwo Jima for a hero's welcome, Lovell's voice-over describes the cause of the explosion, and the subsequent careers of Haise, Swigert, Mattingly, Kranz, and himself. He wonders if and when mankind will return to the Moon.

In a near future, many are addicted to Avalon, a virtual reality shooter. In Avalon, solo players or parties raid levels populated with AI-controlled enemies and opposing players. Winners are rewarded with experience points and in-game money, which can be exchanged for cash, allowing skilled players to make a living. As their brains interact with Avalon directly, it places mental strains on players, and sometimes rendered them catatonic. Ash, a famous player, plays solo since her party disbanded. After a Class A mission, the GM (Game Master) warns her of the next level's danger, and suggests that she join a party. The next day, a Bishop-class character breaks Ash's record time on the same mission. Ash tries but fails to learn about him or his avatar. As she leaves the game terminal, the Bishop player watches her. Ash runs into former teammate Stunner. Murphy, their former leader, went after a hidden NPC in Avalon, a young girl nicknamed "ghost," and became comatose. The girl is allegedly the only gateway into the rumored Special A, a mission where players cannot "reset" (a mechanic allowing people to abort mission without their avatars being killed). Players who went after "ghost" never woke up from the game and became "Unreturned". Ash and Stunner visit Murphy at a hospital, where someone looking similar to "ghost" watches her. At home, Ash searches for words regarding Avalon, and the ghost. The search leads her to the "Nine Sisters", another Arthurian legend reference. Upon entering Avalon, Ash receives an invitation to a meeting, and is ambushed by griefers. They lured Ash there to rob her equipment. After she overpowers a player, the griefers' leader reveals that only the real Nine Sisters – Avalon's creators – know how to access Special A. They are interrupted by an attack helicopter which kills most of the players. Due to a lag, its missiles teleport in front of Ash. She "resets" and leaves Avalon, saving her avatar. On the return home, people around Ash seem immobile, with the exception of a dog. At home, after preparing a meal for her dog, she realizes that it has disappeared. She hears the helicopter flying past. The next day, Stunner meets Ash. He relays a rumor that the ghost appears when there's a high-level Bishop present on the battlefield, so Bishop players are sought out by parties seeking to enter Special A. Before becoming an Unreturned, Murphy was a Bishop player. At her house, Ash is visited by the Bishop player. He offers to form a party with her and she accepts. Ash arrives at the game terminal and tells the receptionist that she plans to enter Special A to look for Murphy. Despite warnings from the receptionist and the GM, she enters the game. There, Ash meets the Bishop player, whom she suspects is working for the Nine Sisters. Stunner arrives, revealing that he was helping Bishop recruit Ash all along. The party confronts the Citadel, a boss. Stunner, Bishop and his summoned dummy players distract the Citadel, while Ash attacks its weak point. After the Citadel is destroyed, Stunner spots the ghost. He is then shot by an enemy. Before being forced out of Avalon, Stunner mentions the only way to kill the ghost. Ash kills the ghost, turning it into a gateway. Ash steps into the gateway and disappears. Ash "wakes up" from the game booth, which is put in her apartment, wearing civilian clothing and without equipment. Bishop contacts her, saying that she is in Class Real. The only way to exit Avalon is to complete the objective: defeat the Unreturned staying there. Ash takes a provided gun and proceeds to her destination, an Avalon-themed concert by the Warsaw Philharmonic Orchestra. En route, she moves through a bustling world, unlike the previous levels and the real world. At the concert hall, Ash sees Murphy, and they walk outside to talk. As she confronts Murphy about his decision to stay in the game, he states that he prefers the "reality" within Avalon. Ash mortally wounds Murphy, who urges her to stay, then disappears. Ash enters the now empty concert hall, and sees the ghost on the stage. Ash trains her gun on the ghost, who smiles. The words "Welcome to Avalon" appear on screen.