The aim is to come up with a spoiler for a well-known film* using exactly four words, without naming the film or any of the characters or actors in it.
The idea is to try to do it with subtlety and creativity, such that it doesn't actually act as a spoiler for anyone who hasn't seen the film (because they won't be able to identify which film it is), but anyone who has seen it should be able to recognise it (though it might require some thought).
Play the game to see some examples.
* You can, of course, extend this to other media, but I think films work best for this game, as they have the greatest commonality of experience - that is, more people are likely to have seen a particular well-known film, than have read a particular well-known book or seen a particular well-known play. Of course, if you play this game in a room full of Eng.Lit professors, then books might be the better choice. Either way, the default media should be agreed in advance, and any deviation from it should be announced.
Because it seems to be the right number for this game. When I first came up with the idea it was going to be 3 words, but I think 4 gives a bit more flexibility to be subtle. Also, it's the number of words in the archetypal spoiler: "The butler did it."
I've made a very simple online version of the game. Play it here. You'll be able to add your own soon.
Conceived and developed by Ben Wheeler @ Uniq Code Ltd <firstname.lastname@example.org>
Many thanks to my Facebook friends who enthusiastically embraced the idea and contributed entries.