Dexter Morgan is a police forensics expert, family man, and serial killer. Luckily he confines his murderous instincts to those criminals who have avoided justice, and he avoids capture by rigorously following rules laid down by his adoptive father, a policeman who realised Dexter was "different". The show never flinches away from the ugly truth, that Dexter is a sociopathic murderer who just happens to prey on bad people. He's always one mistake away from being a monster, and he knows it.
Previously, season five was a little disappointing, as it ended without properly confronting the consequences of what had happened during that season, and during the excellent season 4; the supporting characters were also a bit mishandled. However, season six is clearly determined to make up for this.
The best seasons of Dexter have a strong central enemy, and this season features the excellent Edward James Olmos as a reclusive academic obsessed with the Apocalypse, and Colin Hanks as his impressionable young sidekick, perpetrating a series of murders that they believe will bring about the End of Days. It could have been a bit cheesy, but instead it is handled brilliantly, with a series of macabre murderous tableaux that confuse Miami PD as they attempt to solve the murders. Dexter is, of course, closer on the trail, but he has troubles of his own. Dexter is now a single father, struggling with the fear that one day his son will be confronted with the murderous truth about his dad. It plays as a psychological battle between the killers, much as season 4 did with Trinity, and this brings out the best in the show.
In addition, the supporting characters are handled very well this season, as the writers throw spanners in their personal life. Debs in particular is given plenty to do, and gets some strong development. There are some interesting new characters too, including Masuka getting an intern and a new detective who reminds me of Doakes from the early seasons. La Guerta gets back to what she does best - playing dirty in internal police politics - while Batista has a quiet but competent season. I found Quinn a bit annoying though.
One excellent episode brings back some characters from previous seasons to tempt Dexter away from his code, and the show uses that to set up a brilliant twist a couple of episodes later - the writing is really good, particularly towards the end of the season.
This isn't the best season of Dexter - I still rate seasons 1 and 4 as the best - but it's very good, with some great performances, a twisty plot, and a sense that the show knows where it is going. And I guarantee the last scene will leave you wanting to know what happens next.