Oliver Ines narrates his life from aboard Talos, the spacecraft carrying him and three crew members on a ten-year mission to Europa, one of Jupiter's moons. His account alternates between memories of Earth and mission log entries that chart the crew's psychological and physical deterioration in deep space.
Oliver is born on January 28, 1986, the day the Space Shuttle Challenger breaks apart seventy-three seconds after liftoff. He arrives blue and silent in a hospital near a rural English village before his cries finally reach his terrified father. His earliest vivid memory is of the summer of 1995, when he is nine. His mother sends him to deliver plums to Mrs. Tan across the village, but a girl his age named Philly answers the door. Philly agrees to reveal her full name only if Oliver helps her search for the New Forest cicada, a species that spends nearly a decade underground before emerging for a brief final spring. They find no cicadas but form an immediate bond beneath Mrs. Tan's sycamore tree. The next morning, a note slipped under Oliver's door reads: "It's Philomena."
They spend the rest of that summer in Mrs. Tan's garden. Oliver shows Philly his bedroom, whose walls and ceiling are covered in glow-in-the-dark space wallpaper, a detail later cited in a
Time profile as the origin of his cosmic ambitions. His father takes him for a ride in a restored Mini and tells him over a beer that he could be an engineer, someone who makes things "greater than the sum of their parts." When Philly's parents collect her at summer's end, she makes Oliver promise not to search for the cicadas without her. He watches her leave, recognizing in her loneliness something of his own. Soon after, a developer razes Mrs. Tan's property, and the two children lose touch for years.
The narrative interweaves these memories with mission logs. Oliver, Commander of the PHOENIX mission, describes his crew: Shane Moore, a Royal Air Force (RAF) pilot and Oliver's closest friend; Lucia Bianchi, a physicist; and Dominic Lewis, a pilot and engineer. With no contact with Earth for years, Oliver assigns busywork to combat depression and monitors morale. Shane, usually the group's emotional anchor, grows quiet and asks whether anyone would reconsider joining the mission. Lucia says no, arguing it transcends personal comfort.
On Earth, Oliver enrolls in mechanical engineering at Imperial College London in 2004 alongside Jimmy Lovett, a fellow top student from the village. He feels intimidated by articulate classmates, particularly Lea Mann, who challenges the environmental costs of space exploration. When Jimmy confesses he would rather return to the family farm, Oliver encourages him to go, advice that clears the way for Oliver to receive the Ormond Scholarship. Jimmy leaves London; Oliver and Lea receive the award. The influential Professor Whitley tells Oliver that what distinguishes exceptional students is "ruthlessness," adding that sacrificing family and friends "won't be too much of a problem" for him.
Oliver briefly dates Lea, who warns him against joining the military. For his master's thesis he writes about nuclear reactors, aware the technology serves the military but believing all scientific progress is inherently good. He joins the Royal Navy as a submariner in 2008, serving on HMS Valiant, a ballistic submarine carrying the UK's nuclear deterrent. Months at sea with no outside contact teach him to socialize and transform him.
At a Christmas party hosted by Shane, Oliver encounters Philly for the first time since childhood. She is studying biology at University College London (UCL), her hair tipped with electric blue. They rekindle their friendship. At Christmas 2012, Oliver's father reveals that his mother was diagnosed with breast cancer months earlier. Oliver, who had requested that bad news be withheld during deployments, recognizes the ruthlessness of having made himself unreachable. In February 2013, Philly reveals she has broken up with her boyfriend Josh and wants to be with someone "whose absence I feel." Oliver wants to kiss her but remains mute. It is not until autumn 2016, when Philly visits Oliver's Glasgow flat, that they confront their feelings. Philly describes her "Grand Theory of Life": Life alternates between transition years of underground growth and action years when everything happens at once, using the cicadas as metaphor. She tells Oliver he is impossible to read, and he admits he has wondered about being more than friends. They kiss for the first time on his worn-out sofa. The next morning, Oliver discovers she brought a toiletry case: She had planned to stay.
They marry in the village church in late summer 2017, with Shane as best man. During their honeymoon in Greece, Oliver receives a call from Mark Massey, founder of NovaTech, a private space company developing nuclear propulsion technology. Mark offers Oliver an engineering position on Cilix, a secret probe being built to survey Europa. Oliver accepts. Two years later, Philly encourages him to apply to NovaTech's astronaut program. Oliver, Shane, and a third candidate named Ludo are selected from nine thousand applicants.
On October 10, 2022, Oliver launches to the International Space Station (ISS) with Philly, three months pregnant, pressing her hand to the glass separating them. He performs his first spacewalk and sees Earth's sunrise from 420 kilometers above. After returning, their son Tommy is born. When Cilix confirms a liquid ocean beneath Europa, Mark and Aurélie Gaulthier, head of the European Space Agency (ESA), offer Oliver command of PHOENIX: ten years with no Earth contact, six to reach Europa and four to return. Philly declares it a clear no. Oliver agrees, but Mark's words about immortality lodge in his mind.
Weeks later, Oliver tells Philly he wants to accept. She is furious: "We're your duties, Ollie." In the weeks before departure, she tells him she can no longer see him the same way. She invokes the cicadas, unspotted since 2000, and tells him they are done. Oliver visits his ailing mother one last time. She touches his hair and says, "Buckets of jam."
On Talos, the crew discovers wreckage confirmed as Pegasus, the spacecraft from the failed HEPHA mission that disappeared in 2016. Oliver pushes to investigate, but Lucia objects, accusing him of wanting glory. Shane urges Oliver to think about Philly and Tommy, and Oliver relents. A micrometeorite then strikes the main engine deck. Dom reveals persistent abdominal pain, likely from radiation exposure, so Shane performs the required spacewalk in his place. Outside, they see Saturn, Jupiter, and Europa. A water leak develops inside Shane's helmet, blurring his vision and disabling his radio. Oliver drags Shane toward the airlock, but Shane loses consciousness and drowns in his suit.
On Day 2190, Talos lands on Europa's ice. Oliver takes the first step but cannot speak landing words without Shane. In the cold storage room where Shane's body is kept, Oliver hallucinates Shane's voice telling him there is nothing left on Earth.
After quarantine, Oliver faces a press conference where Mark attributes Shane's death to human error, not equipment failure. Backstage, Lucia accuses NovaTech of sending them up with poorly tested equipment, arguing the radiation shielding failed, causing both Shane's death and Dom's illness. Oliver stays silent; Lucia calls him a coward. Privately, Mark reveals Dom's cancer has spread and discloses plans to mine Europa for resources. Oliver recalls Lea Mann's warning about becoming a pawn.
Oliver returns to the village, where his father, now in his eighties, lives alone, Oliver's mother having died seven years earlier. His father shows him a scrapbook of every article about the mission and expresses regret about having been too strict, worrying his severity drove Oliver away. Oliver assures him it did not. He visits Liv, Shane's partner, in Margate. She tells him Shane left Oliver two hundred pounds from an old bet about the HEPHA mission, and that Shane will only ever be the sidekick in Oliver's story.
Oliver visits Philly, now a professor at UCL. Tommy is with Philly's parents. She asks: Was it worth it? Oliver says he does not know. She tells him they should have been his greater purpose. She cannot forgive him but wants him to have a relationship with Tommy. As he leaves, she hands him a letter. On a park bench, Oliver reads it. Philly describes Tommy trick-or-treating in an astronaut costume, visiting Oliver's mother's grave, and a peacock butterfly landing above her name. She writes that for the first time, instead of anger or sadness, she felt sorry for him.
In the final mission log, Oliver narrates Talos's descent to Earth. Dom drifts in and out of consciousness. Lucia mutters prayers. As the spacecraft shakes violently, Oliver hears a shrill, high-pitched noise beneath the chaos, barely discernible but undoubtedly there, not unlike the sound of the cicadas that called to him in his youth.