The story is intermixed in a series of flashbacks of Calvi's early life up to the age of 17 until shortly before he is murdered. Calvi, in his plush Rome penthouse he hurriedly packs his suitcases with money and a gun, preparing to leave. He flies out in the middle of the night on a private jet making his getaway.
Checking into London's Chelsea Hotel, he lays on his bed remembering back to 1979 when it all started, when he was at the top of his game and Chicago Archbishop, Paul Marcinkus came into his office, complaining about missing Vatican bank funds that Calvi was handling in offshore banks he had set up. Marcinkus gives Calvi a stern warning to fix the situation or else.
At UK Police Headquarters on June 18, 1982, the day after Calvi's body was found. Porter comes into Headquarters to meet with the Chief Police Commissioner to discuss sending him on a fact-finding mission to Rome to investigate the Calvi murder, and why he was killed. Much to their surprise, Italian police have already deemed the crime a suicide. The Commissioner has chosen Porter because he's familiar with Italy and speaks the language. Arriving at Rome's Fiumicino airport, Porter notices men following him. He leaves in a taxi and is tailed by a black car all the way to his hotel, the Intercontinental, atop the famed 'Spanish Steps'.
The story cuts back to 1979 when Archbishop Marcinkus is called to The Vatican and being questioned by a group of Cardinals who want to know why Vatican money and Mafia funds are being laundered through offshore banks set up by Roberto Calvi. Marcinkus assures them all's fine and the Cardinals turn a blind eye as long as the Vatican isn't openly implicated.
Marcinkus then visits the secretive "P-2" Grand Masons Lodge in Rome, headed by the notorious mob boss, Licio Gelli. A meeting has been called to Order amongst 12 top Mason members, and Marcinkus is asked to explain why Calvi's Bank Ambrosiano is being examined by investigators. Marcinkus knows it's about missing funds and tells them he'll create a diversion for auditors and get to the bottom of it.
The story moves back to present day, 1982. Porter is at Rome's busy Police Headquarters. He's teamed with veteran Italian police Inspector, Marco Danzi. They are both interrogating a captured Mafiosi soldier named, Francesco Mannoia, who refuses to cooperate. Porter returns to his hotel to meet his lovely wife Alana who's just flown in from London. He's tells her about his investigation and involvement of the P-2 and Mafia, and she's naturally concerned about his welfare.
In the meantime, Marcinkus meets with some obvious gangster types, who he tells to keep an eye on Porter's every move and to deter him anyway they can. The Pope summons Marcinkus to a meeting and expresses his dissatisfaction with the way the police inquest is going on the Calvi Case. Marcinkus assures him he's handling it.
The following evening Porter and his wife Alana are having dinner outside, at a plush Rome restaurant, when a huge bomb goes off nearly killing them. Safe back in their hotel room again, Porter decides to send Alana back home to London for her own safety.
One night in his hotel room, Porter sees an investigative journalist on RAI TV talking about the corruption surrounding the Calvi Case and the P-2 Mason lodge involvement. Porter goes through his police files and studies the info and activities of the P-2.
The following day at Rome Police HQ, Inspectors Porter and Danzi continue their interrogation of Mannoia. Mannoia finally breaks and tells them word on the street is, a man named Francesco Di Carlo, was the hit man hired by P-2 Grand Master, Licio Gelli. He also tells them that Calvi was killed for siphoning off Mafia and Vatican money into the millions. Danzi is disbelieving when Mannoia mentions that the Pope was aware what was going on, using both the Bank Ambrosiano and Vatican for the laundering of mob money.
When Porter leaves his hotel the next morning, he notices the black car following him through the heavy traffic. Suddenly two men get out with guns and approach Porter's car, and a big chase and gun battle erupts, injuring bystanders. Police arrive just in time to save Porter. Inspector Danzi arrives on the scene and asks Porter to give up his gun--"They can't have foreign nationals in cowboy shootouts on the streets of Rome". Porter gives up his gun and in so doing, leaves himself basically defenseless. Danzi advises him to go home and give up the Calvi investigation.
Porter returns to his hotel to pack, when his wife, Alana, suddenly returns to the hotel unexpectedly and urges him not to give up and to continue his mission: To get to the bottom of the Calvi Affair. She promises to stay by his side whatever happens.
Porter goes to Bank Ambrosiano to question the new Bank Director, Guisseppe Calo, about the case and where he thinks the missing money went, but is stone-walled and given the runaround. In the meantime, Alana goes out shopping when she befriends a German tourist woman, Monica Kleinman, who has just had her purse snatched by a guy on a Vespa motor scooter. She asks Alana to help her find a police station where she can report the incident. As Alana leads her down a dark street, a van swoops and Monica and her boyfriend, Mario Flavoni, shove Alana in the van, kidnapping her.
Porter, unaware what has happened, goes to The Vatican to confront Marcinkus. He is told by monks he is not there, but then runs into him as he's leaving a meeting. Marcinkus is rude and uncooperative and ends up slapping Porter across the face. Marcinkus informs him he is untouchable because of his Vatican diplomatic immunity.
At the Rome Police HQ, Porter is told by Inspector Danzi that a call was received from kidnappers who have his wife, and to lay off the case or else. Porter is floored by the news.
At a country house outside Rome, Alana is bound and being molested by both Monica and Mario Flavoni.
Porter returns back to his hotel in bad shape, worried about his wife. As he climbs the steps to the entrance, a man comes out of nowhere behind him. He turns to confront the person, and discovers it's the RAI TV journalist, Marino Marti. Marti wants to share information on the Calvi murder. They go to Porter's room to discuss the case. Marti warns Porter that no one can be trusted. The people involved in the murder of Calvi go all the way to the top.
Porter decides to work solo, doubting and afraid to trust anyone. He goes to see Graziella Crocher, Calvi's past secretary and known mistress, hoping she can shed some light on the case, regarding Calvi and his associates. She's afraid to talk, but does point the finger at Archbishop Marcinkus.
Shortly after talking to Porter, Marti leaves work and takes a taxi to his home, but the taxi driver takes him to a quiet place and pumps two bullets into him.
Next day Porter gets a call from Inspector Danzi, who tells him not only about Marti's murder, but that Mannoia has hanged himself in his jail cell, and lately he also learns from the TV news that Graziella has supposedly "jumped" to her death from her 5th floor apartment. Porter's devastated, as all witnesses or informers are methodically being eliminated.
Back in his hotel, Porter receives a voice-muffled call telling him if he wants to see his wife alive again, to be at the Colosseum at midnight. He goes to the Colosseum as instructed, where a hitman is waiting to kill him. He manages to over-power the guy and get away. The hitman chases him down and is about to put a bullet into Porter, when Inspector Danzi shows up in the nick of time and kills the thug.
Porter, saved by the arrival of Danzi, is about to thank him when suddenly Danzi turns his gun on Porter. Porter, in shock, listens as Danzi calmly explains the system of corruption has been going on in Italy for Millennia, and no one, least of all Porter, will ever change it. Danzi is about to shoot Porter when he escapes into the dark Colosseum. A major chase ensues. He leads Danzi into a construction area of loose stone and brick where Danzi trips and falls to his death. Porter searches for Alana and finds her alive, but in bad shape in a dungeon storeroom of the Colosseum. The police and ambulance arrive and rush Alana to a hospital.
Convinced their lives are in great danger and no one can be trusted, they make plans to leave Italy.
Before departing, Porter makes one last attempt to gain access to the infamous P-2 Lodge. Porter overpowers the guard at the door and sneaks inside unobserved. In a dark, candle-lit chamber with long flowing curtains, he witnesses the macabre initiation ritual of a new Mason member, being performed by Grandmaster, Licio Gelli. Porter searches and finds the hidden list of names of P-2 members linked to the government and Vatican in a hidden wall panel that Marti had told him about.
Porter exposes himself and confronts Gelli, demanding the truth about the Calvi murder and who was responsible. Gelli, convinced he now has Porter trapped in the lodge with no escape, tells him everything. The black-robed and hooded Mason members surround Porter, and he pulls out a gun. Porter opens fire, killing several of them and escapes into the lodge, running through the dark candle-lit halls, setting fire to the curtains on his way out. Exiting to freedom, he turns and sees the entire Lodge ablaze.
Porter and Alana make a quick getaway heading out of Rome through the rugged mountain roads when another car pulls out trying to run them off the road. Going 100 mph, Porter crashes through a road construction site barrier to avoid a large dump truck in their path. The car careens out of control and swerves over a steep cliff, sailing into the Tiber River, some 200 meters below.
The black car that was following them screeches to a halt and two men get out. One is a policeman, the other a Rome Magistrate. In the Magistrate's hands is an official file marked, "CALVI". The Magistrate crosses himself, looking at the wreckage below, then throws the file into the raging Tiber and a watery grave.
The Ending is a text dénouement explaining the outcome of the Calvi Case listing all the real life characters, and that to this day, no one was ever indicted, charged, or found guilty of the murder of Roberto Calvi, (AKA The Devil's Banker) or loss of the millions of dollars from The Vatican and Bank Ambrosiano.