If you have seen seasons 1 and 2 then you know who Dexter is and what he does.
The 3rd season is excellent,as D continues to evolve in unusual ways.His "normal" persona is forming a strong attachment to the children of his girlfriend.So much so,that when there is the hint of a threat to them(in the form of a man D discovers is a pedophile),D breaks all the "rules" that govern his murderous behaviour(only kill convicted killers who got off on a technicality) to dispatch the pedophile before he can act.He also discovers his kind,caring,girlfriend is pregnant by him and is faced with the prospect of fatherhood!At the office,he steers his cop sister through a few cases to boost her chances of promotion,but the most intense part is when he is actually "caught" by an ambitious A.D.A at the scene of his latest victim.The ADA, does not turn in Dexter as he mistakenly believes that D has dispatched the murderer of his own brother.Thereafter, a strange relationship develops between the 2,that D ultimately must resolve.
Anyway ,lots of episodes that take D further.I was given the U.S. version by a friend,so am not sure what will be on the UK version.But the U.S. version has loads of extra's on disc 4.In general,I find the U.S. versions have a lot more on them than those sold in the UK,so if you can,and are a real fan,try to get the U.S. versions of DVD's.