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.
It's probably fair to say that season four of Dexter was a triumph, with an excellent plotline and a truly shocking ending. The challenge for season five was how to follow that. The first episode opens minutes after the shocking finale of season four. Dexter had spent much of that season trying to see a way to combine a family life with his murderous impulses, and thought he had managed to deal with the threat of the Trinity killer and find a way forward. However, the last scene of the series destroyed all that, and now he has to try and deal with the consequences.
Some of the smaller problems are that Detective Quinn becomes suspicious of Dexter and hires someone to investigate him, and that Dexter's precarious relationship with his step-children is wrecked. However, the main plot sees Dexter find someone he might be able to share his issues with properly, and is a decent attempt to follow season four without copying it. The new characters, and the murderous plot surrounding them, are good, and the casting is excellent. Nothing meets the heights of the Ice Truck Killer or Trinity, but it's a worthy addition to the show. However, whereas Trinity's psyche was explored fully last season, the new main threat finishes the season without their mindset really being explained.
The use of other characters in Dexter has always been a bit patchy. His sister Debs doesn't really evolve much, and her new relationship in this season doesn't make much sense. The relationship of Batista and La Guerta has much melodrama but very little change, and Masuka is underused.
Ultimately, this series shies away from confronting some big problems. Harrison is shuffled off whenever he's not convenient, Dexter gets remarkably little trouble from the Trinity investigation despite having left a trail that could expose him, and the big question of how Dexter can live like an ordinary person is ignored. The end of the season doesn't quite hit the reset button, but very little is really solved, sorted or changed. Still, it's a good season of Dexter, and Dexter is very good television indeed.