An "explosive chain of events" leads to Corbett and McQueen becoming persons of interest to AC-12. We don't know what those events are just yet, but season four ended with our three key officers of AC-12 viewing a huge collection of evidence that pieced together the previous seasons' events so far, all linked by a balaclava-clad criminal gang.
The network of "bent coppers" is even bigger than anyone thought possible, and the level of police corruption goes even deeper than they ever imagined.
We can expect to learn more about the regulars, too, as series creator Jed Mercurio told the BBC he wanted to "look at the personal lives of all the regulars in series five" — particularly as they took a "backseat" to Roz Huntley in season four.