International Immobiliare was a European real estate company. Its properties were worth over $6 billion by the 1970's, making it the largest landowner in the world. The Vatican owned a 25 percent interest.
Michael Corleone bought a large block of stock during the 1970s, eventually becoming the company's largest single shareholder. In 1979, he expressed interest in buying the Vatican's stake for $600 million in the form of a deposit in the Vatican Bank, which would have given him controlling interest. He had plans to turn it into an international conglomerate. Despite fierce opposition to the Corleone family's aquisition of this company, the company's American directors voted to accept Michael's tender offer.
However, the Immobiliare deal was really an elaborate swindle concocted by company chairman Licio Lucchesi. He had stolen a fortune from the Vatican Bank with the help of bank head Archbishop Gilday and accountant Frederick Keinszig, and intended to use Michael's "investment" to cover his tracks. When it looked like the deal might actually succeed, Lucchesi tried to stall the deal by making Michael promise to do business with him in return for his support. His stalling was inadvertently helped by Pope Paul VI's illness; under the provisions of the Lateran Treaty, the Pope had to personally approve the deal. Lucchesi also conspired with Corleone caporegime Joey Zasa and Don Altobello to have Michael assassinated. The plan was temporarily delayed when Zasa was murdered by Vincent Mancini.
Michael eventually caught wind of the swindle, but didn't know that Lucchesi was also trying to have him killed until Vincent, while spying on Altobello, was introduced to Lucchesi. Vincent is named head of the Corleone family, and as his first official act has the conspirators wiped out-- but not before his cousin Mary was killed by Don Altobello's assassin, Mosca.
Immobiliare is based on its real life counterpart and on Banco Ambrosiano. In the late 1960s Immobiliare was bought by Charles Bluhdorn's conglomorate Gulf+Western, which also owned Paramount Pictures. Bluhdorn, as president of Paramount, played a decisive role in the production of the first two Godfather films.