Searching for solutions to the pressing issues of overpopulation and global warming, Norwegian scientist Dr. Jorgen Asbjornsen from the Edvardsen Institute invents "downsizing" — a groundbreaking process that shrinks organic material irreversibly. Established in the 1950s with the revenue derived from mustard gas production during World War II, the institute aimed to utilize scientific advancements for the betterment of humanity. Recognizing overpopulation as a pivotal threat, they discovered that downsizing could offer an unlikely yet effective solution.
As one of the pioneering test subjects, Jorgen is thrilled to learn that the process reduces humans to a mere height of about five inches (12.9 cm), drastically cutting down on both their resource consumption and waste output. Five years later, the global buzz surrounding the process reaches a climax when the experiment's results are unveiled at a major conference.
Dr. Andreas Jacobsen, who collaborated with Jorgen on this innovative procedure, elaborates that the downsizing process is swift and relatively painless, with only mild discomfort from minor dental adjustments needed during the procedure. Test subjects are lightly sedated and typically recover within a couple of hours.
The test subjects enjoyed their time in a secure 7 by 11 meter gas-permeable habitat, shielding them from the elements, animals, and pests. In a span of four years, only a single garbage bag was filled with their non-compostable waste. The institute envisions a long-term transition spanning 200 to 300 years from a "big" to a "small" society, even marking the birth of the first "small" baby.
Fast forward a decade, and we meet Paul (Matt Damon) and Audrey Safranek (Kristen Wiig), a married couple grappling with financial instability in Omaha. The worldwide economy is in crisis, losing billions due to reduced consumer spending linked to downsizing. Industries like construction, automobiles, and housing are feeling the strain as homes are left empty when individuals downsize.
At a college reunion, they reunite with Dave (Jason Sudeikis) and Carol Johnson (Maribeth Monroe), who have both embraced the irreversible downsizing process. While the inventors promote downsizing as a green solution with less waste, Dave argues that it significantly enhances one's quality of life by increasing the value of money.
Intrigued by the downsizing possibilities, Paul and Audrey attend a promotional event for Leisureland, a community designed for the small. They meet a "small" couple, Jeff and Laura, who enthusiastically share how their entire diamond necklace cost just $83, the same amount they typically spend on food for two months. Their lavish lifestyle in Leisureland offers luxurious 12,000 sq. ft. homes for a fraction of the price they would pay in their previous life. With Paul’s savings translated into a whopping $12.5 million in Leisureland currency, they decide to commit to the downsizing process.
As Audrey's parents bid farewell, the excitement of moving to Leisureland looms. However, they are faced with a 1 in 225,000 chance of complications during the procedure. Paul is ushered into the men's section, anticipating a reunion with Audrey shortly after the treatment.
To his dismay, Paul later receives a heartbreaking call from Audrey, revealing her decision to back out of the procedure at the last minute, leaving him to navigate his new life solo. A year down the line, Paul finalizes his divorce from Audrey and tries to adapt to his new existence, working as a telemarketer for Lands' End after having let his occupational therapist license lapse, believing he wouldn’t need it after downsizing.
While attending a birthday party, Paul expresses his regret about his choice to downsize during a conversation with Dave. Just after, he breaks up with his girlfriend and attends a gathering hosted by his neighbor Dusan.
The next day, Paul encounters Ngoc Lan Tran (Hong Chau), a Vietnamese political activist downsized against her will. She was the only survivor of a human smuggling operation to the U.S. and had undergone a leg amputation due to an infection sustained during transit. Initially intending to assist Lan with her prosthetic, Paul inadvertently damages it, leaving her unable to work.
In return for Lan’s kindness, Paul aids in her cleaning business and helps gather food for the needy in their community. Dusan tries to lure Paul away to Norway, where a significant event awaits, offering them lucrative prospects. Lan, however, insists on joining due to her newfound fame from her past, which even attracted attention from Dr. Asbjornsen.
On their journey to Norway, Paul and his companions encounter Dr. Asbjornsen and his wife while sailing through a fjord. The doctor shares a grim warning about humanity's impending fate due to the feedback loops of Arctic methane emissions that could eventually lead to extinction.
In the first downsizing colony, the doctor reveals his contingency plan: a vast vault built into a mountain meant to cradle humanity's future until conditions on the surface stabilize, potentially thousands of years later. While Dusan and his friend Joris Konrad express skepticism about the catastrophe timeline, Paul feels drawn to enter the vault. Yet, when he reaches the closing door, he has second thoughts and decides to return home with Lan, Dusan, and Konrad.
Back in Leisureland, Paul embraces his role in aiding Lan as they provide essential support to the marginalized individuals living in the surrounding slums, choosing a path of service and community over isolation in a vault.
