Rob Brydon plays Keith, a hapless cuckold who addresses us via a camcorder set up in his mini-cab. The Marion and Geoff of the title are his estranged wife and her new lover, though as Keith--who never fails to perceive a bright side to his utterly dismal existence--says, "I don't feel I've lost a wife, I've gained a friend."
Through his monologues, we learn that Keith has a room in a student house where banging techno is played day and night; that in order to make the journey to see his two boys, he must make an overnight journey from London to Cardiff by car; that his only friend is a tollbooth operator (though the operator doesn't seem to know it) and that, although he's been driving a minicab for a while, he's yet to pick up a fare.