Heat is the story of two obsessive careerists who doggedly prioritize their work over every meaningful personal relationship in their lives, until it consumes them almost completely. LAPD Lieutenant Vincent Hanna (Al Pacino) and expert thief Neil McCauley (Robert De Niro) run into each other at work - unfortunately, they operate on opposing sides of the law. They should exist in diametric opposition to each other. But part of the excellence of Michael Mann's film is that they don't. After Hanna realizes that his tracking efforts have led him to be tracked in turn by McCauley, he develops a begrudging respect for his charge.
This leads to an iconic diner scene (Pacino and De Niro's first shared scene ever), wherein Hanna buys McCauley a cup of coffee as a bit of a ceasefire. Hanna has nothing he can pin on McCauley yet, so they get together to talk. And they find out that, despite the one vocational hurdle, they have more in common than not. But ultimately, unfortunately, the job is the rub.
"You know, we are sitting here, you and I, like a couple of regular fellas," Hanna bottom-lines it, conceding his weird respect for his counterpart. "You do what you do, and I do what I gotta do. And now that we've been face to face, if I'm there and I gotta put you away, I won't like it. But I tell you, if it's between you and some poor bastard whose wife you're gonna turn into a widow, brother, you are going down."
"There is a flip side to that coin," McCauley counters. "What if you do got me boxed in and I gotta put you down? Cause no matter what, you will not get in my way. We've been face to face, yeah. But I will not hesitate. Not for a second."
Unfortunately, a scenario comes to pass wherein their mutual professional admiration may give way to the demands of these lifers' chosen jobs. Let's just say not everybody in that diner scene gets out of Heat alive, although even in the end, the survivor gently sees off his dying counterpart.