As the clock relentlessly ticks towards disaster in 1962, Dr. Calvin Webber—a scientist known for his unconventional ideas—unfurrows his brow over the impending threat of nuclear war with the Soviet Union. In a daring act born from his paranoia, he constructs a hidden fallout shelter beneath his backyard. When his pregnant wife, Helen, finds herself swept up in Calvin's doomsday visions, they make the decision to retreat to their underground refuge just as the Cuban Missile Crisis reaches a boiling point. On a fateful day, disaster strikes when an F-86 Sabre crashes into their house, leaving them scrambling for safety—an event that leads authorities to abandon their property under the assumption of calamity.
Down in their bunker, Helen gives birth to Adam, who grows up isolated, conditioned by his father to possess exceptional knowledge. His childhood revolves around 1950s pop culture; shows like I Love Lucy and The Honeymooners form the backdrop of his early years, while tunes from Perry Como and Dean Martin linger in the air. Meanwhile, on the surface, the neighborhood undergoes a dramatic transformation—a diner opens in 1965, where a young Melker earns his first paycheck as a soda jerk. As time flows on, the diner evolves into a pizzeria and later a punk venue called Purgatory, reflecting the descent of the area into disrepair.
Fast forward to 1995, and Melker has hit rock bottom, becoming a jaded, middle-aged drunk living amid the dilapidated remains of Purgatory. By 1997, the door to Calvin's shelter finally opens, revealing a neighborhood devastated by years of neglect. While Helen yearns to reconnect with the outside world, Calvin is content to remain hidden away, steadfastly resistant to leaving. As their supplies dwindle and Calvin's health deteriorates, Adam takes the responsibility upon himself to venture out for food.
Fortuitously, Adam encounters Eve Rustikov while attempting to sell his father's vintage baseball cards at a hobby shop. When the shop owner attempts to swindle him, Eve intervenes, prompting Adam to feel gratitude towards her. Motivated by a desire for guidance, Adam, entirely oblivious to the financial implications, offers Eve a remarkable $1,000 per week to assist him not only in navigating life but also in finding a wife adhering to his mother's eccentric requirements: a woman from Pasadena, California, who must be “not a mutant.” Enter Troy, Eve's flamboyant and stylish roommate, who supplies Adam with invaluable advice and gives him a much-needed makeover.
With Eve at the helm, the trio embarks on a quest for a suitable partner for Adam at a retro 1940s swing nightclub. As Adam garners interest from numerous women—including Eve's rival, Sophie—he unknowingly stirs feelings of jealousy within Eve. In an attempt to create distance, Eve reconnects with her ex-boyfriend, Cliff, who provokes Adam into a physical altercation. To Eve's astonishment, Adam's unforeseen boxing skills make their presence known. In the aftermath, she rekindles her romance with Cliff, only for Troy to deliver the alarming news of Adam's departure with Sophie. Upon his return, Adam expresses his true intentions to take Eve as his wife, "underground," which leads Eve to ask him to leave.
Finding solace at the same nightclub, Melker, now leading a following, figures prominently in Adam's journey. Seeking comfort, Adam returns to Eve's house, discovering her alongside Dr. Nina Aron and her assistant, eager to have him committed. Complying initially, Adam later escapes, instructing Eve and Troy to collect his belongings and settle his hotel accounts. In his absence, they stumble upon a trove of retro treasures, including investment certificates and personal belongings deemed worthless by Adam’s father. This discovery leads them to understand Adam’s true narrative and that he is indeed sound of mind.
While Melker's cult congregates in their underground haven, Calvin prepares once more to seal his family's fate. As Calvin locks out the outside world again, Eve catches a glimpse of Adam lingering outside the nightclub, leading to a heartfelt reunion that culminates in Adam introducing her to his parents. Amidst this connection, Calvin and Helen sense a kindred spirit in Eve and consent to pause for two months, granting Adam and Eve time to strategize. During this interlude, the couple liquidates their assets, funneling the proceeds into creating a rustic escape that mirrors their former home, lost to the whims of fate, while they lovingly restore a vintage 1960 Cadillac convertible.
As they breathe new life into Melker's pub, transforming it into a nostalgia-driven '50s hotspot, they work to dismantle the misinterpretations surrounding Adam’s prophetic assertions. It is during this time that Adam discloses a groundbreaking revelation to Calvin: there wasn’t an atomic war; rather, the Soviet Union peacefully dissolved, marking the end of the Cold War. Though Calvin’s incredulity echoes in the air, he resolves to construct another fallout shelter, with Eve watching him with a blend of apprehension and curiosity as she fiddles with her engagement ring.
