The Wanderers

Helen Kane, a 53-year-old retired NASA astronaut, visits the Japanese branch of Prime Space Systems Laboratory, where CEO James "Boone" Cross recruits her for an unprecedented program. Prime Space, a private multinational corporation, plans to send the first human crew to Mars. Before the actual voyage, the company will conduct Eidolon: a 17-month simulation of the entire mission, including the outbound trip aboard a spacecraft called Primitus, 30 days on the Martian surface, and a return voyage aboard Red Dawn. Helen, who has spent her post-NASA years feeling irrelevant on Earth, writes her name on the wall and commits.

Helen's crewmates are Sergei Kuznetsov, a 45-year-old Russian cosmonaut, and Yoshihiro "Yoshi" Tanaka, a 37-year-old Japanese astronaut. Sergei leads the outbound transit, Helen commands the Mars expedition, and Yoshi takes the return. During a rest day in Japan, Sergei discloses his divorce from his wife, Nataliya; his sons, 15-year-old Dmitri and the younger Ilya, will move to New Jersey with their mother. Helen and Yoshi offer support, signaling they will back his framing if Prime questions his stability.

The novel alternates between the astronauts' experience and the lives of those they leave behind. Helen's adult daughter, Mireille Kane, is a massage therapist and aspiring actress in Los Angeles who goes by "Meeps." She oscillates between resentment and fantasy, fearing the mission will become the defining story of her own life. Yoshi's wife, Madoka Tanaka, a businesswoman who sells robotic caregivers, fears she is "slipping away" into being defined solely as the wife of the man who went to Mars. Sergei's older son, Dmitri, begins a secret relationship with Robert, a college music student in New York, slipping away during the Saturdays he escorts Ilya to ballet class.

At Prime's Utah facility, a cognitive scientist named Luke joins the Eidolon Observation team, a group that monitors the astronauts' psychological and physical states throughout the simulation. All three astronauts answer assessments with identical deflections, revealing their skill at concealment rather than anything about their inner lives.

Eidolon launches on the same day Prime sends Red Dawn II, an actual rocket, to Mars. The first weeks aboard Primitus bring cascading technical problems, sleep deprivation, and relentless demands. Helen begins experiencing involuntary "mind-slips": intrusive memories of her father, who had a traumatic brain injury when she was nine and spent years in a permanent vegetative state. She records a strained video for Mireille on the anniversary of her late husband Eric's death.

Mireille pursues acting work while reading her mother's emails with longing and frustration. Madoka, alone in a Stockholm hotel room, confesses to her demonstration robot, PEPPER, that she does not want motherhood and that Yoshi loves a version of her she does not recognize. Dmitri's relationship with Robert deepens through furtive visits, though he conceals his real age and family. A dorm sign-in sheet inadvertently reveals his first name to Robert, who had known him only as "Misha."

A pivotal disruption occurs when the crew wakes to nausea and disorientation. Yoshi realizes, recalling Galileo's supposed words "Eppur si muove" ("And yet it moves"), that Primitus is not on a static platform. Prime has been physically manipulating their environment, shaking the crew's assumptions about what else the company might be doing.

When the Chinese lunar mission Weilai 3 suffers a catastrophic launch failure, killing all four crew members, the news reaches the astronauts just after their simulated Mars landing. A flashback reveals the weekly hospital visits of Helen's childhood, where the family sang "Happy Birthday" beside her father's bed.

The crew spends 30 days on simulated Mars. During a surface excursion, a dust devil erupts beneath Helen. Her faceplate cycles through colors while static fills her ears. For a suspended moment she thinks "Dad?" and then decisively thinks "No," recognizing her father is truly gone. She emerges transformed and elated. Sergei, however, experiences something different: his simulation fails for a few seconds, revealing a living sky with the Martian moon Phobos transiting the sun. When the simulation returns, the colors feel dead.

After launching from simulated Mars, Sergei convenes a secret meeting in the lavatory, the only space without confirmed audio surveillance, and writes on a whiteboard that he saw the real Mars. Helen and Yoshi offer alternative explanations, but Sergei cannot dismiss the possibility that Prime secretly sent them to the actual planet. During the return voyage, he privately constructs conspiracy theories, reviewing circumstantial evidence while recognizing the hallmarks of paranoia in his own reasoning.

Yoshi is destabilized by a moment on Mars when Helen unselfconsciously asked him to vacuum dust from her body after removing her compression suit, revealing "what a large thing it is to be Helen, what infinite space she is." He acknowledges that on Mars he lost the ability to imagine Madoka. When Prime announces a backup crew, Yoshi makes an uncharacteristically cruel remark about one member being "a Helen but without the tragedy," stunning Helen, though she chooses not to be hurt.

On Earth, Luke develops feelings for Mireille but recognizes the ethical constraints of his role. Madoka has lunch with Mireille in Los Angeles, where Mireille reveals her technique for crying on cue: imagining her family singing "Happy Birthday" beside Helen's father's bed. Madoka responds that only a human can say a "sad yes" when asked about love, and that "a sad 'I love you' is art." Dmitri, forced to reveal his real age and family to Robert after a confrontation in Central Park, shares his first real kiss. Madoka rescues a stray dog and resolves to stop waiting for Yoshi or herself to become who they should be.

Each astronaut composes a letter they never send. Sergei writes to Dmitri, confessing he knows his son is gay and begging Dmitri not to hate the real man behind the simulation of strength. Yoshi writes to Madoka, comparing their marriage to Pluto and its moon Charon in mutual tidal lock, each showing the other only one face, and admits he has loved her "incorrectly." Helen writes to Mireille, acknowledging she wanted space more than motherhood but asking her daughter to accept a love that "doesn't look like any other kind of love but it's not a bad love."

Before landing, the crew holds a final meeting in the lavatory. Sergei says what he saw was "not so different" from the simulation, "like taking off sunglasses," but still amazing. He declares he will not tell Prime anything it does not need to know. Helen admits she has a practiced voice she uses when struggling, which she calls "PIG": Polite, Interested, Good-humored. The astronauts touch each other gently and agree they are "fundamentally sound" even though "everything breaks in space."

As Red Dawn reenters Earth's atmosphere, Helen experiences genuine fear of dying for the first time, a sensation she welcomes as evidence of how much she now has to lose. She fixates on one desire: to hold her daughter. She recognizes that afterward, she will probably want to return to space just as much as ever. The parachutes deploy, the sky appears, and the crew returns to gravity. The final passage merges Helen's voice with a collective yearning: the blue sky, the people waiting, the knowledge that this feels real.