Lionel (Pacino) and Max (Hackman) promise Max’s sister and her friend to return to Denver after stopping in Detroit to see Lionel’s estranged wife and the baby she was pregnant with when he left. They then plan to go on to Pittsburgh to pick up money Max has (supposedly) deposited in a bank there.
In Detroit, Lionel calls first rather than simply showing up and is told by his terribly distressed and jilted wife that she has remarried (true) and that their son never came to term (a lie told to hurt him).
He lies to Max and tells him that he has a son and they go to celebrate, during which Lionel has a nervous breakdown and slips into a catatonic state.
The film ends as Max uses his last cash to buy a roundtrip ticket to Pittsburgh to get his deposited money, the implication being that he will return to Detroit for Lionel.