A one page summary of Shakespeare's wonderful comedy can be found here: http://www.enotes.com/muchado/36454
More detailed discussions of each act can be found in the study guide as well.
If you want a shorter summary, the play is a comedy about the trials that two couples go through as they get together. Claudio falls instantly for Hero, but John breaks them up with lies and it takes the whole play for them to get back together. Beatrice and Benedict fight instantly, and it takes tricks by outsiders to get them to fall for one another, which they do. So, two couples, both lovers, both tricked by outsiders, both end happily.