Surprise or spoiler? If you don't want to know whodunnit, don't click on that site

It's one of the great theatrical whodunnits. For 58 years, the identity of the killer in Agatha Christie's The Mousetrap has been kept a secret. The twist is of such magnitude, the audience is requested at the end of each performance not to reveal the ending to their friends. And incredibly, despite this being the longest running play in British theatre history, the strategy has worked. If you haven't seen The Mousetrap, chances are the Monkswell Manor murderer remains an enigma: you can travel to London's theatreland, the West End, ticket in hand and safe in the knowledge that the ending will genuinely be a surprise. Until now, that is.

All it takes now is one visit to Wikipedia's Mousetrap page, and 58 years of secrecy are quickly blown apart. Last week, Matthew Prichard - Agatha Christie's grandson - pleaded with the online encyclopedia to remove the section that gives away the controversial but quite brilliant denouement. "My grandmother always got upset if the plots of her books or plays were revealed in reviews and I don't think this is any different," he said in The Independent On Sunday. "I think it is a pity if a publication, if I can call it that, potentially spoils the enjoyment for those people who go to see the play. It's not a question of money or anything like that. It's just a pity."

But Wikipedia won't budge. They argue that Prichard's plea is akin to asking a librarian to remove the book The Mousetrap from the shelves, just in case someone flicks to the end. "It's exceedingly easy to avoid knowing the identity of the murderer: just don't read it," a spokesman said. In actual fact, it's not that easy to avoid, because Wikipedia doesn't give any warning that the storyline is about to be revealed. There's no "spoiler alert" in big letters - the usual convention for material on the internet that could impair the enjoyment of an unwitting browser. So you could happily be checking how many performances of The Mousetrap there have been (more than 24,000), or who was in the original cast (Richard Attenborough) and suddenly happen upon the identity of the murderer. And, with one mistaken glance, the mystery - and the fun - is ruined.

It's easy to blame the internet for all this, of course, but our hyper-connected world has undoubtedly made it increasingly difficult to avoid plot spoilers. With every action and thought seemingly broadcast via Twitter and Facebook, to go into a cinema with no idea of the twists and excitements that lie ahead requires a monk-like dedication to avoiding social media. And woe betide any fans of television dramas who have a social life, and have to record an episode to watch later. It's almost impossible to get through the following day without seeing or hearing something that reveals the contents of the episode.

So how do you see a film with fresh eyes, beyond booking cinema tickets for the advance preview? It's difficult, but it certainly helps if the film in question establishes popular and critical acclaim. In which case, an invisible internet army of Spoiler Police seem to mobilise out of nowhere, deleting comments and generally shouting in CAPITAL LETTERS every time anything remotely revealing is written.

Christopher Nolan's Inception is the most recent example of this: even the stars refused to talk about the specifics of the plot during interviews. For many, it is still possible to buy a ticket for "that film about dreams" without knowing much else besides. But how long will that last? The general rule of thumb is that as a story ages, it becomes more acceptable to talk about its twists. So if there's anyone here who doesn't know why Bruce Willis can't seem to communicate with his wife in The Sixth Sense, then, well, you don't take much of an interest in movies. But back in 1999, it was a huge cultural faux pas to discuss this in public.

The real ethical minefield, though, surrounds film adaptations of widely read books, as the popular film critics Simon Mayo and Mark Kermode found out. Mayo mentioned (SPOILER ALERT) the demise of a certain well-loved headmaster in Harry Potter and the Half Blood Prince, to a huge radio audience. He imagined - naively as it turned out - that 65 million sales worldwide meant that people knew the storyline and so he could discuss it.

It is not always the case that spoilers are the mean-spirited actions of killjoys who want to ruin everyone else's fun. Inception is a film so dense and intelligent that naturally people want to talk about it as soon as they leave the cinema. And who hasn't watched an episode of Lost, not quite understood it, and headed straight for the fan forums to try to make some sense of it? "I think people know enough about 'spoiler warning' etiquette for it to be possible to read what you want to read and avoid the rest," says Dan Kaiser. And he should know: Kaiser runs The Movie Spoiler, a website that publishes spoilers to all the major films, even paying for contributions from the public. In fact, Kaiser's site doesn't just stop at revealing key twists, the entries give a blow-by-blow synopsis of the entire storyline. Incredibly, 30,000 people visit The Movie Spoiler every day: that's an awful lot of people who don't like a surprise with their popcorn.

"Well, I've found that many people come to my site for reference, particularly if they didn't understand something in the film ? or fell asleep in the middle of it," Kaiser explains. "You know, I often receive e-mails actually thanking me for giving the plot away because it meant people didn't have to waste their time actually seeing the movie, but could still be involved in all those water-cooler discussions with their friends."

Nevertheless, it is a relief to find out that even Kaiser didn't open the Inception spoiler submitted to him until he'd seen the film himself. "That doesn't happen very often," he says. But he has learnt a lot about the way the film industry works via his site and the 23 years he previously spent as the manager of a cinema. "A movie with a twist always generates much more attention than a movie without one. It never fails."

And, as Agatha Christie's grandson has probably learned this week, complaining about plot spoilers creates much-needed publicity all of its own, too. Still, The Mousetrap deserves such attention and to have its twist so jealously guarded. You'll never guess it... and we're not about to tell you whodunnit either.