The neglected Platonic dialogue Euthydemus is peculiar in many ways. It is, apparently, an extensive catalogue of bad arguments by disgraceful sophists; but its complex composition suggests that this focuses attention on the shape and nature of argument—attention that some think Plato is incapable of giving. He uses the idiom of games, and of seriousness and play, to provoke reflection on logical and syntactic structure and their normative features; but to see how he does so we need to consider the complex background of the fiction of a Platonic dialogue, and its use of surprise and humour. Comparison with the bbc Radio 4 game ‘Mornington Crescent’ might help.