Plot

A physically perfect but innocent man goes in search of his long-lost twin brother, who turns out to be a short, womanizing, small-time crook.

Julius and Vincent Benedict are the results of an experiment that would allow for the perfect child. Julius was planned and grows to athletic proportions. Vincent is an accident and is somewhat smaller in stature. Vincent is placed in an orphanage while Julius is taken to a South Seas island and raised by philosophers. Vincent becomes the ultimate lowlife and is about to be killed by loan sharks when Julius discovers that he has a brother and begins looking for him.—John Vogel <jlvogel@comcast.net>

A physically perfect human experiment who lives in a paradise finds out that he has a twin brother on his 35th birthday. But doesn't know that they are the complete opposite. However, they were both lied to about what really happened about their mother, and must find out the truth.

Many years ago, an experiment was conducted to produce a perfect human being. Unfortunately, there was a serious problem. The zygote split unevenly leaving two children - one with strength, brains, personality and love and the other one with what else was left! Both children were raised separately at different locations until one day, the two unite and help each other find their long-lost mother.—Michael Feller <reb@magna.com.au>

Julius (Arnold Schwarzenegger) and Vincent Benedict (Danny De Vito) 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 Julius died at birth, and was not even aware that Vincent ever existed. Julius was informed that his mother died in childbirth. Vincent was raised in an orphanage and learned from a letter in his file that his mother had abandoned him. Mary Ann had gone on to become a successful artist. While successful, the genetics program was considered a failure and shut down because of the conception of the twins, one inheriting the "desirable traits," and the other getting the "genetic trash."

Vincent (DeVito) was placed in an orphanage run by nuns in Los Angeles, California while 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 small-time criminal, involved in shady business deals and car theft and in debt to notorious loan sharks, the Klane brothers for $20,000. He is also a womanizer and a wise cracker 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 naive about the real world his more worldly brother inhabits, he bails Vincent out of jail and pays to get Vincent's car out of impound, but is afterward snubbed by Vincent, who leaves him at the impound lot. 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-again-off-again girlfriend, Linda Mason. Knowing little about women, Julius doesn't understand the flirtatious advances of her blond 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 their 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.

While driving a stolen Cadillac he plans to sell to his chop-shop contact, Vincent discovers via a cassette that in the trunk is a secret prototype fuel injector for jets, which is to be delivered to a rival industrialist in Houston, Texas in exchange for five million dollars. Posing as the contract delivery man, Mr. Webster, Vincent intends to use the money to pay what he owes to the Klane brothers. At Julius' insistence, 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, not knowing that she had given birth to twins, and fearing they were land speculators. Little does Vincent know is that Webster himself is tracking them. This man has already killed his contractors because they saw his face, in order to preserve his identity. At one point earlier on in the film, Webster breaks into Vincent's house to 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 becomes more bitter, 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 (despite earlier promising Vincent she'd wouldn't tell), and Julius embarks on another journey to track his brother down. Vincent eventually delivers the stolen property to Beetroot, but as Vincent is about to return home with the money, Beetroot and his assistant are 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, feeling his brother's presence, 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, 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 realizes that the two "comedians" who visited her were in fact her long-lost sons. She pays Traven a visit and punches him right in the nose for "stealing her family."

Julius and Vincent marry the Masons and use the $50,000 reward money to start up a legitimate consulting business, using Julius' knowledge and Vincent's questionable business savvy. Mary Ann 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, with their mother and Professor Werner completing the big, happy family.