Sometime in the future, Earth has been ravaged by pollution, natural disasters and rising sea levels, making it largely uninhabitable. A woman (Emelie Garbers) works on board the Aniara, a luxurious spaceship that takes passengers from Earth to Mars in three weeks. Her job involves working as a "Mimarobe" within the Mima, an artificial intelligence designed to evoke the viewers' experiences of Earth's lush, verdant past through a totally immersive virtual-reality experience that taps into the participants' memories and emotions.
In the first week of the Aniara's voyage, the ship suddenly veers off course to avoid a collision with space debris. Some of the debris pierces the hull and hits the ship's nuclear reactor, setting off an imminent meltdown and forcing the crew to eject all of the ship's fuel. This results in the ship having no navigational control, no propulsion, thus no ability to resume its original course. Captain Chefone promises the passengers and crew that they will be able to resume the trip to Mars once the ship passes a celestial body, which should happen in no more than two years. The Mimarobe's roommate, the ship's astronomer, later reveals to her that this is a lie and that there is no possibility of resuming their course.
Soon, the Mimarobe finds her usually unimportant job becoming more popular and necessary than ever, as passengers use the Mima as an escape from their current situation. After three years, the Mima becomes one of the most important functions necessary to keep calm on board the ship. With so many people bringing their memories of Earth's decline to the Mima, as well as their anxieties surrounding the current situation on board the Aniara, the Mima becomes overwhelmed and self-destructs, dying by suicide. Though the Mimarobe had asked the captain for a month of rest for the Mima, she is blamed for the machine's malfunction and is imprisoned. Isagel, a pilot and the Mimarobe's lover, is also imprisoned following a physical conflict with Captain Chefone regarding the punishment of the Mimarobe.
By the fourth year, mass suicides and developing cults lead the Mimarobe and Isagel to be granted release and reassigned to work. They join a fertility cult dedicated to the Mima, and soon Isagel becomes pregnant after an orgy. She has depression during her pregnancy and is tempted to end the child's life after he is born. The Mimarobe wants to build a "beam-screen", a projection device acting as a mimic of Mima to alleviate Isagel's and the other passengers' depression, but Captain Chefone forbids her from doing so. He instead orders her to focus on educating the children, in hopes that one or more of them may discover a way to return them to Mars.