Julius (Arnold Schwarzenegger) and Vincent Benedict (Danny DeVito) are unique fraternal twins, the unexpected outcome of a controversial genetics experiment. This experiment, aimed at creating the ideal child, involved the sperm donations of six different men. Their mother, Mary Ann Benedict, was led to believe that Julius had perished at birth and was completely unaware of Vincent's existence. Similarly, Julius was misinformed about his mother's fate, being told she died during childbirth. Meanwhile, Vincent was raised in an orphanage, discovering through a letter in his file that his mother had abandoned him. Mary Ann eventually became a successful artist, leaving behind a life shrouded in secrets.
Despite the experiment receiving criticism and being deemed a failure, it yielded two drastically different results: one twin inherited exceptional traits, while the other ended up with what was described as “genetic trash.” Vincent grew up in a Los Angeles orphanage, whereas Julius was nurtured on a remote South Pacific island by Professor Werner, one of the scientists behind the project. There, he developed into a man of remarkable intelligence, exceptional physical strength, and a strong moral compass, mastering twelve languages and excelling in various academic disciplines. Surprisingly, Julius remained oblivious to Vincent's existence until he turned 35.
In contrast, Vincent faced a very different upbringing. As soon as he was old enough, he escaped the orphanage, embracing a life of petty crime involving dubious dealings and car thefts, all while being deep in debt to ruthless loan sharks known as the Klane brothers. His chaotic lifestyle included womanizing and a keen pursuit of wealth, which ultimately led to his arrest over unpaid parking fines.
Discovering the existence of his brother, Julius travels to Los Angeles with pure intentions. Armed with his intelligence but lacking worldly experience, he bails Vincent out of jail and retrieves his impounded vehicle, only to be dismissed by Vincent, who leaves him behind. However, Julius is undeterred and manages to track Vincent down, leading to a confrontation at his workplace where he bravely intervenes when one of the Klane brothers, Morris, threatens Vincent.
As their paths intertwine further, Julius encounters Vincent's volatile girlfriend, Linda Mason. Naïve about female interactions, he struggles to interpret the advances of Linda's sister, Marnie. Despite this confusion, Julius gradually finds himself falling for Linda. Vincent, still grappling with feelings of abandonment, reveals that he possesses a document from the orphanage that confirms their mother is alive, yet remains disinterested in seeking her out. Intrigued, Julius discovers their mother's address and sets off to learn more about her, where he meets one of his fathers, who explains the details of the genetics experiment and directs him to another scientist, Mitchell Traven, in hope of finding their mother.
Amidst the chaos, Vincent inadvertently comes across a secret prototype jet fuel injector stashed in a stolen Cadillac he's planning to sell. With hopes of using the cash to pay off his debts, he devises a plan to masquerade as a delivery man named Mr. Webster. Under Julius's insistence, the brothers embark on a cross-country road trip to find Traven, who ultimately reveals that their mother is residing in an art colony near Santa Fe.
Their journey takes a perilous turn when the Klane brothers locate them at a bar, intent on revenge. However, the brothers manage to fend them off one last time. When they arrive at the colony, they receive devastating news: their mother has passed away. Unbeknownst to them, the woman who delivered this news is actually their mother, who is oblivious to their existence, mistakenly thinking they are land speculators.
As events unfold, Vincent wrestles with feelings of betrayal and bitterness, which strain his relationships with Linda and Julius. Determined, Julius sets out to track down his brother, who proceeds to deliver the stolen injector to the dangerous industrialist, Beetroot McKinley. Just as Vincent is about to claim his pay, a villain named Webster, who had been secretly hunting them, kills Beetroot and turns his focus to Vincent when Julius arrives.
What follows is an intense chase where Julius valiantly intervenes, leading to a final showdown with Webster. As the danger escalates, Vincent reluctantly returns to confront Webster, but thanks to Julius's quick thinking, they manage to eliminate him. Ultimately, the brothers reconcile, and Vincent returns the stolen items, though he quietly pockets a million dollars for himself.
Their newfound fame soon alerts their mother, who realizes that the two men she met were her long-lost sons, leading to an emotional reunion with Traven for her. Embracing their new life, Julius and Vincent marry Linda and Marnie respectively, using their reward money to launch a legitimate consulting business that combines Julius's intellect with Vincent's shrewdness. In a heartwarming conclusion, their mother tracks them down to their office, resulting in a touching reunion, and in the end, both brothers joyously welcome pairs of twins with their wives, embodying a beautifully united family alongside Professor Werner.
