Of course you should tell them the whole story. I'd give them a brief summary of the plot, then watch the cartoon (Animated Tales). Shakespeare wrote to be performed, not to be read... Younger students are much less afraid of Shakespearean English than KS4. You need to decide what you want them to get out of the play and then study a few key scenes in depth. Check out some of the discarded SATS preparation resources that are still available online. I've got a DVD copy of the brilliant Patrick Stewart version which was on stage last year in Chichester and then transferred to the West End. I think the whole (3 hour!) play would be a little heavy for Year 8, but it might be worth investing in a copy (from Amazon) to show them the odd clip of key scenes you choose to use. It's only available in the USA, so you'd have to check that you can get some Region 2 software. The Trevor Nunn version with Dame Judy and Sir Ian also helps bring the language alive, in my experience. Students quite like the whole association of Gandalph and M being married!