Lamberto is a cardinal in the Roman Catholic Church, and the man favoured to succeed the terminally illPope Paul VI as head of the Church. He is visited by Michael Corleone on the advice of Don Tommasino, and Michael tells him of his swindle at the hands of Frederick Keinszig, Licio Lucchesi, and Archbishop Gilday. Lamberto encourages Michael to confess his sins; Michael is initially reluctant, but eventually gives in under the Cardinal's gentle prodding. He breaks down in tears when confessing that he ordered the murder of his brother Fredo. Upon the conclusion of the confession, Lamberto tells Michael that it is just that he suffers for his sins, yet absolves him and tells him he still has a chance for redemption.
Upon the death of Paul VI, Lamberto is elected the new Pontiff; he takes the name John Paul I upon his accession. A moral, thoroughly honest man, he immediately calls for an investigation into the activities of the Vatican Bank and requests a meeting with Keinszig, the Bank's chief accountant. However, Keinszig has left Rome with a large sum of money and several documents. Fearing that their corruption will be exposed, Keinszig, Lucchesi, and Gilday plot to murder the Pope; Gilday poisons his tea, killing him in his sleep.
The character of Lamberto and the film's depiction of the events which lead to his murder are based upon one of the conspiracy theories surrounding the sudden death of the real-life Pope John Paul I, Albino Luciani. Like the Lamberto character, Luciani was discovered dead in his bed in 1978, 33 days after his election to the papacy. Various theories, such as the one outlined in David Yallop's 1984 book, In God's Name, have suggested that Luciani was murdered because he was investigating and planning reforms for the Vatican Bank, in light of the Banco Ambrosiano scandal.