This game bills itself as 'collaborative storytelling' rather than full-blown role-playing, with scope - indeed need - for three players: the classic 'buddy pair' of cops and a third person playing 'The Beat'. This last individual represents the setting and plot and plays everybody else who appears in the game, but is NOT a game master per se. The cop players may choose to be veterans, rookies, rogues or even a K-9, best each chooses a different one (unless you want two veteran cops or possibly two rogues!).
Once roles are decided, you need to determine the setting by deciding which city you're operating in, the name of the unit our cops work for and even a few details about the Chief; as well as determining a criminal organisation and its leadership. That gives The Beat something to work with, whilst the other two players get to grips with their cop characters, again with a series of questions to help you flesh them out. Then they'll need to decide how good they are at various skills necessary to a cop's life: bureaucracy, combat, dialogue, investigation and pursuit. Rank them in order from 1 to 5. There are a few other things as well but every mechanical advantage has to be described in narrative during the game to take effect.
The way that The Beat differs from a classic GM is that, despite devising and running the plot, he shares narration with the two Cop players, rather than them solely talking about what their characters are doing. With use of die rolls and the skills previously determined, they take hold of part of a scene describing the action in its entirety - what they are doing AND what everyone else is doing as well. Interestingly, a key difference between this and classic role-playing is how essential die rolls and game mechanics are to driving the story forwards, rather than merely resolving tasks when someone is attempting something for which the outcome is open to question. The person who takes charge at any point during the game is determined by passing a physical token, the Badge, to them. At certain points - like setting the initial scene - the Badge should be in The Beat's hands, but most of the time it is just passed clockwise around the table so that everyone gets a turn.
There are plenty of examples to show how the different rules can be applied, and if collaborative storytelling and police procedurals are your thing, this is a good vehicle with which to tell a few shared stories.