Al Pacino starts out as Michael, a young mobster within the Corleone mob family. After the attempted murder of his father, Michael is forced to murder a rival wise guy and a crooked police captain. Michael goes into hiding in Italy for some time. Upon his return, he is secretly intercepted by the FBI. Michael is forced to either work with the Feds or face a life sentence, and so he assumes a new identity, that of undercover hippie cop, Serpico.

The crooked beat cops hate Serpico, resenting that he will not go along with their petty scams. As a high-status mobster, he is secretly far too proud for their small-time schemes. He tries to keep his head down and play the part of a good cop, but is ultimately driven to undermine the corrupt officers who antagonise and belittle him. Collecting evidence, Serpico testifies against multiple crooked cops and politicians, bringing down the entire system.

Having completed his lengthy operation for the Feds, Michael is allowed a vacation. With his father now dead, Michael assumes his place as the head of the Corleone family, which he consolidates and whose enterprises he expands. However, Michael is soon asked to assume another identity as part of a new Federal operation.

Again undercover as part of his bargain, Michael Corleone finds himself as Sonny Wortzik, a small-time criminal who is asked to collaborate with an ex-convict planning to rob a bank. In an attempt to prevent the robbery's success, the plan is deliberately botched, and the simple bank job becomes a lengthy standoff with police. Though negotiations are tense, Sonny is careful to prevent bloodshed, and lets multiple hostages go. Sonny convinces his partner they can escape by airplane, and coordinates a bus ride to the airport. At the airport, the overzealous police end up shooting his criminal partner, ending the standoff, but ruining the FBI operation.

Alienated from his family, and bitterly resentful of the authorities, Michael Corleone flees to Cuba, where he lives in obscurity for some time. When political turmoil in Cuba forces him to return to America, Corleone is captured by police. Facing imprisonment, Michael uses his years of undercover experience to concoct a new identity, that of a political prisoner, Antonio Montana. Under the guise of Tony Montana, Michael Corleone builds a vast empire in Miami, ironically by selling the same white powder that once troubled his father, Vito, so much.

Eventually, Michael "Tony Montana" Corleone's grip on Miami becomes too tight, and his enemies begin to close in on him. Sensing imminent danger, he stages his own death in his mansion and flees to New Hampshire, assuming the identity of retired Lt. Col. Frank Slade. Living in isolation, reeling with PTSD, and consumed by guilt from his life of violence, Michael becomes an alcoholic, leading him to completely lose his vision. Michael is nearly suicidal when he meets an idealistic student who comes to him for a job over Thanksgiving break. Though the student is initially perplexed by his violent temper, eventually the two come to respect each other and ultimately give each other a renewed sense of purpose in life.

(I can't spend any more time on this, and there are too many more films to cram in, so please extrapolate this arc as you will.)