In a post-apocalyptic world devastated by a catastrophic solar flare, the film follows Tom Hanks as Finch Weinberg, a former robotics engineer who has managed to survive underground with his loyal dog, Goodyear. The story unfolds in the ruins of St. Louis, where Finch and Dewey, his homemade robot, venture out to gather supplies. During one such trip, they narrowly escape a sudden wind and sandstorm, highlighting the dangerous and unpredictable environment they live in. As Finch heads home, another massive sandstorm approaches, forcing him and Dewey to seek refuge quickly. Fortunately, Finch manages to hide, and he and Dewey survive the storm's wrath.
Finch's world is one marked by solitude, illness, and a desire for survival, as he is battling what appears to be radiation poisoning caused by the solar flare. His goal is to find a way to care for Goodyear once he is gone, knowing his own time is limited. To that end, he has been working tirelessly on a robot he has built, giving it a vast amount of knowledge and the ability to communicate, albeit with some initial roughness. After some early setbacks, the robot begins to respond more human-like, nodding and developing a partial speech pattern voiced by Caleb Landry Jones. Finch is proud of his creation, but Goodyear isn't entirely pleased with the newcomer, sensing something unfamiliar and potentially threatening.
As Finch keeps an eye on the weather, he detects a colossal superstorm headed their way, forecasted to last for about 40 days—a period they cannot survive confined with minimal resources. Recognizing the urgency, Finch prepares to leave swiftly, packing up Goodyear, Dewey, and the robot into an RV. Their destination is San Francisco, a city the character believes might offer better safety and possible help, especially since they want to avoid encountering other desperate survivors who could pose a threat.
On their journey, Finch attempts to program Jeff, the robot, further, especially as its data transfer remained incomplete before their hurried departure. They make a few stops, including an abandoned movie theater where Finch amusingly shows Jeff how to make popcorn. Later, as a tornado threatens, they hurriedly secure the RV to the ground and take cover inside, witnessing the destructive force pass overhead. After the storm subsides, the robot affectionately adopts the name Jeff, and they continue their voyage.
Throughout their travels, Jeff struggles to interact smoothly with Goodyear, sometimes amusing Finch but also frustrating him, especially when Jeff nearly causes the RV to drive off course. Finch, exhausted and ill, must rest, which leads Jeff to ask why they don't travel at night instead. Finch explains that the darkness is dangerous because of potential encounters with hostile survivors. One morning, Finch becomes visibly ill, violently vomiting, which Goodyear notices. Demonstrating his growing trust and empathy, Jeff takes the wheel of the RV to help during Finch’s illness.
During their stop at a store, Jeff’s attempts to help accidentally cause Dewey to get caught in a bear trap, destroying the robot and devastating Finch. The loss hits him hard, and he angrily scolds Jeff. As night falls, they realize they are being followed by another vehicle. Finch directs Jeff to hide the RV under a bridge, but the vehicle's height causes it to break off. They then push further and wait for the other survivors to leave. Finch shares a haunting memory of the early days of the apocalypse, recounting how people turned hostile—like a mother instructing her daughter to shoot anyone approaching, leading Finch to kill a man who threatened their safety. He also finds out about Goodyear’s origins as a puppy in the backpack of a young girl, a heartbreaking detail that underscores his loss and survival.
Eventually, Finch and Jeff reach a spot with low UV radiation, where Finch can step outside into the sunlight for the first time in years. Dressed in a clean suit, he shares stories from his past with Jeff, who begins to see Finch as a father figure. Finch teaches Jeff and Goodyear to play fetch, adding a poignant sense of normalcy to their bleak lives. However, Finch’s health rapidly deteriorates, and he starts coughing up blood, ultimately succumbing peacefully in his sleep inside the RV.
Following Finch’s death, Jeff performs a small funeral, quietly contemplating their next move. His speech has improved significantly, sounding more human, which signifies his growth. Determined to honor Finch’s wishes and continue their journey, Jeff decides to travel toward San Francisco. He drives himself and Goodyear across the ruined city until they reach the iconic Golden Gate Bridge, where Jeff finds survivor markings and messages indicating other groups of people still alive. Leaving a drawing of himself with Finch and Goodyear, Jeff then walks into the city, hopeful yet cautious, hoping to find allies and a new beginning among the scattered survivors.
