The story begins in the prehistoric era with a Neanderthal family consisting of the father, Thorn, a pregnant mother named Hera, their daughter Lark, and their youngest child, Ebb. The family's stability is shattered when a severe fall leaves Thorn in critical condition; while he barely survives through the desperate efforts of Hera, tragedy continues to haunt them. The children are devastated when Lark discovers that young Ebb has passed away. Shortly after, Hera gives birth to a healthy baby but tragically dies from complications during the process, leaving a grieving Thorn and Lark to raise the newborn child alone.
Eventually, the remaining family encounters a group of strangers. Although they initially attempt to flee in fear, they are captured by the group, who turn out to be friendly Homo sapiens. These humans welcome the Neanderthals into their village, and the two species live together in complete harmony for many years. During this time, Lark marries a member of the community and starts a family of her own. After a long and fulfilling life, Thorn peacefully passes away in his sleep. He is buried holding a cherished family heirloom: a small necklace crafted from an acorn that Lark had played with as a young child.
Fast forward to the year 2025, where Claire, a Canadian PhD student in anthropology at Princeton, is conducting research on the remains of a Paleolithic man. She believes her findings could be game-changing for the field of science. During this time, she is in the early stages of a romantic relationship with Greg, a statistics student. However, Claire struggles with emotional vulnerability, often shunning or avoiding him. As they finally begin to grow closer, Claire receives the heartbreaking news that her mother, Claire's Mom, is dying. This forces Claire to return to Canada to provide care for her mother in her final days. Though she considers ending her relationship to avoid the burden, she and Greg decide to maintain a long-distance connection.
After the passing of her mother, Greg moves to Canada to be with Claire, who continues her research remotely. By 2030, the couple has a son named David, and Claire becomes pregnant again. Many years later, an adult David takes over the research his mother started. He eventually makes a global announcement that the project has succeeded in finding a way to extend human life expectancy far beyond its natural limits. His parents, watching with immense pride, gift him the artifact found with the Paleolithic man: a tool carved in the shape of an acorn. Greg and Claire continue to live happily together into their old age, eventually passing away one after the other.
In the distant future, as the Earth faces its inevitable end, a "last-hope" mission is launched to save the human race. The mission is led by Coakley, a scientist and astronaut who has been biologically engineered for an extended lifespan. She embarks on a grueling 226-year journey to a hospitable planet called Kepler-16b, accompanied only by her AI co-pilot, Rosco. Their primary objective is to gestate and raise embryos kept in stasis to establish a new civilization. Because humanity eventually decided that unnaturally prolonged life was detrimental, these new humans are designed to have standard lifespans.
Disaster strikes when the ship's greenhouse, essential for producing oxygen, becomes contaminated. Coakley and Rosco determine that there are only two scenarios to ensure the embryos' survival, both of which require one of them to die. While both are willing to sacrifice themselves, Coakley eventually agrees to Rosco's plan: the AI will be permanently shut down so that the AI room can be converted into a backup greenhouse. In a heartbreaking scene, Coakley tearfully says her final goodbyes before turning off her companion.
Roughly a century later, as the vessel arrives at its destination, Coakley brings the first embryos to life. She names the firstborn child V and gifts her a small trinket shaped like an acorn, intended to symbolize the enduring legacy of humanity. The ship lands successfully, and the planet is found to be perfectly suited for human life.
Over the following years, a stable and peaceful human colony is established. Eventually, V passes away as an elderly woman, wearing the acorn trinket on a collar around her neck during her funeral. Coakley, who still has many years of life remaining due to her engineering, delivers a poignant eulogy. She reflects on the life of V and the trajectory of the human race, noting how far they have traveled despite the fact that, compared to the vast lifespan of the universe, human existence is as brief as the blink of an eye.
