Rufus MacQuarie, a miner in Alaska, witnesses the moon’s destruction and immediately checks the position of the International Space Station (ISS).

An Arjuna asteroid known as Amalthea has been captured and tethered to the ISS. After years of continual upgrades, the station has become informally known as Izzy. Rufus’s daughter, Dinah MacQuarie, works on Izzy studying Amalthea. Under the command of Ivy Xiao, Dinah uses specialized robots to explore the asteroid and harvest metals. While she works in her remote lab, her father sends her a Morse code message urging her to look at “what used to be” the moon (11).

Dr. Dubois Jerome Xavier Harris, popularly known as Doc Dubois or Doob, attends a fundraising event when the moon explodes. For a moment, he doubts the reality of the sight, until the sudden ringing of phones convinces him that this is “the birth cry of a new age” (14).

Ivy Xiao, commander of Izzy, spends much of her time in the torus, where simulated gravity helps counteract her tendency toward space sickness. Though she and Dinah appear physically and socially separated, the assumption that they dislike one another is a product of rumor. Four hours after the moon’s destruction, Ivy convenes a meeting in the torus with all crew members, including Frank Casper, Jibran Haroun, Konrad Barth, Lina Feriera, and Dinah. In consultation with Aurelia Mackey, NASA’s deputy administrator, the team discusses the potential causes and effects of the catastrophe. Mackey warns them to prepare for incoming lunar debris and announces the suspension of all spaceflights until further notice.

Afterward, Dinah visits Ivy in her office. Using their private shorthand of Q codes (three-letter abbreviations adapted from radio slang), they share tequila and speak informally about Ivy’s upcoming wedding to her fiancé, Cal, as well as about Dinah’s father.

In the days following the event, Doc Dubois becomes a public voice of reassurance. The seven large pieces of the moon are given non-threatening names like “Potatohead,” “Kidney Bean,” and “Scoop.” Initially, the expectation is that these pieces will remain in orbit and eventually re-coalesce. However, at a Caltech “star party” a week later, Dubois witnesses a collision that breaks Kidney Bean into two pieces. Leaving the gathering, Dubois experiences a sudden, unsettling intuition, recalling the feeling of being watched by hyenas in Tanzania. He realizes that he has erred in focusing too narrowly on the cause of the moon’s destruction rather than its consequences.

In a meeting with US President Julia Bliss Flaherty (Julia, or J.B.F.), Dubois warns that the moon’s fragments will continue to collide and break apart, leading to a catastrophic sequence of events: the White Sky, when collisions create a visible haze of debris, and the Hard Rain, when trillions of fragments fall to Earth, destroying all life on the surface. Dubois estimates that the Hard Rain could last up to 10,000 years, forcing humanity to seek survival underground or in space.

The crew aboard Izzy joins the meeting virtually. Konrad challenges Dubois’s conclusions, but Julia affirms that scientists worldwide have reached similar results. Leaders predict that the White Sky will begin in roughly two years. Julia unveils the Cloud Ark initiative, which will transform the ISS into a hub for smaller, swarming modules designed to protect humanity in orbit. NASA director Scott Spalding (Sparky) assigns new roles to the Izzy crew. Dinah resists orders to abandon her robotic research, arguing that mining space resources will be essential, and Spalding emotionally agrees.

A Russian Soyuz ship docks with Izzy, carrying critical supplies (termed “vitamins”) and personnel: Maxim Koshelev; Bolor-Erdene (“Bo”); Yuri and Vyacheslav, scientists scheduled to arrive even before the disaster; and Rhys Aitken, an engineer prone to space sickness. Dinah, worried about cosmic rays damaging electronics, uses her robots to carve a tunnel into Amalthea and secure transistors inside for protection. Rhys, tough but incapacitated by sickness, contributes an idea for using bits of Amalthea to shield the robots from radiation.

On Earth, Dubois watches from Crater Lake, Oregon, as the US President and other leaders worldwide formally announce the Hard Rain to the public. Their speeches focus on preserving humanity’s culture and biological legacy. Mary Bulinski urges citizens to send DNA samples and photos of local lifeforms. Celani Mbangwa appeals to people for stories, music, and videos. Clarence Crouch invokes Noah’s Ark, asking communities to select a boy and a girl to train for places on the Cloud Ark. Dubois reflects on the unspoken truths: Few of these children will survive, and those chosen for space will largely be young women, valued not only for reproduction but also for perceived social steadiness. Returning to a rented lodge, Dubois spends the evening with Amelia, a teacher he met at the Caltech star party. Aware that he’ll be chosen for space while she remains behind, Amelia suggests that he might carry a frozen embryo. They consummate their relationship, acknowledging the inevitability of separation.

On Izzy, Dinah exchanges messages with her father, who expresses pride in her work. Rhys observes their coded Morse communication and then shares that he has collected frozen water from the torus, linking it to Arjuna Expedition director Sean Probst’s keen interest in ice as a resource. He wants Dinah to research ways to keep robots affixed to ice in space, like she has done with magnetic robots that can stick to metallic asteroids like Amalthea. Rhys and Dinah sleep together.

Russia deploys “Scouts” to rapidly expand Izzy in preparation for the Cloud Ark. Living in inflatable Luk pods tethered to the station, Scouts endure hard conditions, spending up to 16 hours a day working in Orlan space suits before crawling through a narrow “Vestibyul” into their pods. The space flight and living arrangements are dangerous, and faulty equipment soon causes multiple fatalities. Ivy decides to store the bodies in an empty section of Izzy and then, when the room is full, ceremonially cremate them by sending them through Earth’s atmosphere. Dinah’s robots aid in establishing Luks, and she becomes intrigued by one of their inhabitants, Tekla. Dinah sends one of her robots with chocolate for Tekla.

At another meeting in the torus (nicknamed the Banana), Ivy reminds the team that 5% of humanity’s time left on Earth has already elapsed. Fyodor, an experienced Russian cosmonaut who has become the de facto leader of Izzy’s new arrivals, dismisses the meeting as a waste of time and voices concerns over the Luk’s defective doors.

Dinah soon realizes that Tekla is trapped inside her pod and tells Fyodor she can help. Working with Ivy, Rhys, and others, she devises a dangerous plan to extract Tekla by aligning her Luk with Dinah’s air lock, but the maneuver nearly fails. Dinah closes the hatch on Tekla’s foot, and when the crew retrieves her, she’s bloodied and injured: “Tekla was in there, drawn up into the prescribed fetal position. A solid mass of red” (105). Despite her wounds (lacerations and petechiae from vacuum exposure), Tekla survives. Dinah remarks that survival depends not on following orders from a doomed Earth but on forging their own rules.