While Going Postal is an adaptation of the thirty-third book in Terry Pratchett's Discworld series, it is not necessary to have any background going in. The books cover a wide variety of characters, and there aren't filmed adaptations of most of them, so Going Postal is approached as a completely stand alone story. Considering that Pratchett's books sell second only to J.K. Rowling in the UK, and that this film is a fairly faithful version, it would be advisable to watch it.
Going Postal centers on Moist von Lipwig (Richard Coyle, Coupling, The Whistleblowers), a con man who is finally caught by the law. After a (purposely) failed hanging, Lipwig is brought before Lord Vetinari (Charles Dance, Game of Thrones, Bleak House), who gives him a choice: fall to his death, or reopen the defunct postal service. Lipwig chooses the latter, assuming he can easily make some money, then escape again to further his various schemes.
What Lipwig doesn't realize is that the post office seems to have a life of itself. Millions of undelivered letters fill the halls, and the letters on the letters have the ability to come alive, showing Lipwig the consequences of his past mistakes. You see, while Lipwig thought forging bonds and such were victimless crimes, he has actually destroyed many families. Faced with what he has done, Lipwig has a serious crisis of faith in who he is, and must decide whether he even wants to continue with his bad habits.
Not that he has much choice. To ensure that Lipwig doesn't take off, Vetinari gives him a golem named Mr. Pump (voiced by Nicholas Farrell, Torchwood) as a parole officer. Mr. Pump knows where Lipwig is at all times, and trying to escape is ineffective.
To make matters worse, there are those who don't want to see the post office reopen and be restored to its former glory. Namely, Reacher Gilt (David Suchet, Agatha Christie's Poirot), who runs the Clacks, a message service using light signals. Gilt has paid an assassin to take out the past four postmasters, and has similar plans for Lipwig.