Julius and Vincent Benedict are fraternal twins; the results of a secret experiment carried out at a genetics laboratory to produce the perfect child, using sperm donated by six different fathers. The mother, Mary Ann Benedict was told that the children died at birth, while the children were informed once old enough that their mother died in childbirth. Accepting this, Mary Ann had gone on to become a successful artist. While successful, the program was considered a failure and shut down because of the conception of the twins, one inheriting the "desirable traits", and the other being the "genetic trash".
Vincent (DeVito) was placed in an orphanage run by nuns in Los Angeles, California whilst Julius (schwarzenegger) was taken to an unnamed South Pacific island and raised by Professor Werner (one of the scientists involved in the experiment) like a modern Doc Savage to become highly intelligent, physically very strong and spiritually pure. He learned to speak twelve languages, and excelled in mathematics, history, science and literature. He was not told about his younger brother until his 35th birthday.
In Los Angeles, with no-one but himself to rely on, Vincent escaped from the orphanage as soon as he was old enough and developed into a shady lowlife, involved in shady business deals and car theft and in debt to the notorious loan sharks, the Klane brothers. He is also a womanizer and a smart aleck with a lust for money. Eventually, his debts catch up with him and he is arrested for unpaid parking fines.
Julius is told about his unknown brother by Werner, and comes to Los Angeles to look for him. Highly intelligent, but extremely naïve about the real world his more worldly brother inhabits, he bails Vincent out of jail, but is immediately afterwards snubbed by Vincent, who leaves him in the prison car park. Julius, however, tracks Vincent to his workplace, where he is being beaten by one of the Klane brothers, Morris, for the unpaid debt. Julius easily bests Morris and earns Vincent's respect and trust. He eventually meets Vincent's on-off-on girlfriend, Linda Mason. Knowing little about women, Julius doesn't understand the flirtatious advances of her blonde sister Marnie (who dislikes Vincent), but eventually falls in love with her. Vincent soon reveals to Julius a document he stole from the orphanage when he ran away that proves that her mother is in fact still alive, yet Vincent, under the belief that his mother abandoned him at birth, shows no interest in tracking her down. Julius, however, finds their mother's address on the document and pays the place a visit. While their mother is not there, Julius meets one of his six fathers, who fills him in on the experiment and informs him of the other scientist, Mitchell Traven, who might know where their mother is.
Using a stolen Cadillac that carries a secret prototype fuel injector, which Vincent is delivering to a rival industrialist in Houston, Texas in exchange for five million dollars which he intends to use to pay the money he owes to the Klane brothers, the two couples go on a cross-country journey to track Traven down. They eventually find Traven in Los Alamos, New Mexico, who tells them that their mother is living in an art colony near Santa Fe. On the way to the colony, they're found at a bar by the Klane brothers, with the intent of killing Vincent, but Julius and Vincent beat them back and fight them off for the last time. Reaching the colony, they're informed their mother died and leave. In reality, the woman who told them of the passing is in fact their mother, but she didn't believe the story, fearing they were land speculators and since she didn't even know she had twins. Little do they know that the real contract delivery man for the injector, Mr. Webster, is tracking them. This man has the uncomfortable habit of killing the people who cross him in his business (including his contractors) if they see his face, in order to preserve his identity. At one point earlier on in the film, Webster breaks into Vincent's house in intercept him, crossing paths with the Klane brothers. In a skirmish, he shoots two of them in the legs but does not kill them.
While Julius seems to accept their mother's death, Vincent however is angry and disappointed, taking it out on Linda and Julius. Vincent storms off, leaving Julius and the girls stranded in New Mexico to deliver the engine to the industrialist, Beetroot McKinley. Linda informs Julius of the engine, and Julius embarks on another journey to track his brother down. Vincent eventually delivers the stolen property to McKinlely, but as Vincent is about to return home with the money, McKinley is shot and killed by Webster, who then turns his attention to Vincent just as Julius arrives. A cat-and-mouse chase ensues and Julius intercepts Webster as Vincent flees, but Vincent, hearing his brother's voice, reluctantly goes back and gives up the money to Webster. But as Webster prepares to kill them both for seeing his face, Julius stalls Webster long enough for Vincent to release a heavy chain onto Webster's head, killing him and burying him in a mountain of chains. Julius and Vincent make amends, and Vincent reluctantly agrees to return the money and the stolen engine to the authorities, but Vincent secretly skims off one million. Meanwhile, the twins' publicity reaches the art colony, and their mother realises that the two "comedians" who visited her were in fact her long lost sons.
Julius and Vincent return the money (Vincent keeps 1 million for himself), marry the Masons, and use the $50,000 reward money to start up a legitimate consulting business, utilizing Julius's knowledge and Vincent's questionable business savvy. Their mother eventually tracks them down to their office and they share a tearful reunion. In the end, both brothers end up having pairs of twins with their respective wives, their mother and Professor Werner as a big happy family.