Unexpected Alien in Bagging Area

Two newlyweds, Bathsheba and Gabriel, are enjoying a drink in their local pub, deep in the Wessex countryside, when they are joined by a being from another galaxy. The alien informs them that they have been Chosen to perform a Pivotal Role in Interplanetary History. Gabriel, being a decent sort of chap, gets another round in, but the couple aren't really interested in the alien's offer (in Wessex, newlyweds have better things to do).
But their hand is forced, and they soon find themselves embroiled in a bitter war between two faraway planets who can't stand each other, because the inhabitants of one have absolutely no sense of humour while the others like nothing more than a good (or, even more, a bad) joke. Things become even worse when it becomes clear that one group of aliens (the humourless lot) has more on its mind than conquering its neighbour - they have their cold, unblinking eyes (all eight of them) on a second planet, earth. Can our unlikely heroine and hero outsmart them and save earth from invasion? Enjoy this slightly bonkers, totally intergalactic but also very British comedy.