This is easily amongst the best of the BBC's audio productions, while it is a very funny production and most of the episodes could be listened to as single productions, each having plenty of stand alone gags, there is also character development and more long running plots or themes.
Each episode features the devil, "old harry" of the title, as he goes about working mischief in the world of man, making his frequent visits to earth, torturing the souls of the damned and confronting, well, what would appear to be existential angst, boredom and burnout for all eternity. I think this is very clever and reflects a bit of real thinking about what would be the consequences of living forever, having virtually unlimited power etc. A lot of the gags revolve around these themes, also some obscure and well researched material from the world religions, myth, legend and such also furnishes good material too.
It would appear that virtually everyone is in hell, that is besides God, Jesus and the arch-angels such as Graham, mistakenly recorded as Gabriel in the bible, it certainly is were everything is going on and often the fates devised as eternal torment are ingenuious or ironic. A great many of those who find themselves in hell dont believe it or are slow to come around to the idea believing that it is absurd, this is part of the charm for the entire production for me, that it portrays a world in which all the wonder, horror and awesome nature of myth, legend and religion, as incredible as it all may seem, is absolutely true, and there is everyones downfall for the most part.
Thematically the devil, his no.2 side kick, the most reprehensible and most virtuous, or otherwise interesting, members of the damned investigate facets of life, death, meaning, redemption, fate and morality which is at once hilarious and thought provoking. Most of it is done with cynicism and irony but there's optimism in play too and just desserts. Very, very highly recommended.