[Spoiler-laden summary: To be fair, you’ve had almost 400 years to read it or watch it.] The “nothing” that there is much ado about in this play is the [non-existent] hanky-panky of Hero, daughter of Leonato and the female lead. As it happens, a couple young and studly bachelors (Claudio and Benedick) roll into town after valorous service in the war. This causes some angst in Don John, bastard (Shakespeare’s word, not mine) brother to the Prince of Arragon, because his stock just plummeted. Don John, thus, devises a plot exploiting one of his minions and one of Hero’s attendants to make it look like Hero is flirting about with a strange man. This is a problem because it’s 1623 and Claudio wants his bride to be able to wear white.
When Hero is accused by some credible [but misled] witnesses including her fiancé, Claudio, she passes out from incredulity. Claudio takes off without knowing her condition. A Friar with a devil on his shoulder suggests Leonato tell everyone that Hero’s heart gave out and she died. Friar Francis is one of a handful of steadfast fans of Hero (as well as her cousin and best-friend Beatrice and – by extension – the man courting Beatrice, Benedick – who’s not so much sure of Hero’s virtue as he is sure that Don John is a jerk.) Sad as it is, Hero’s father and her fiancé are ready to relegate her to skanks-town, but her smart-aleck bestie / cousin Beatrice and the priest each have her / his own idea of how things can be set straight. As mentioned, the friar thinks that if Claudio believes Hero died, the better angels of his nature will make him come and atone for his accusations. Beatrice’s approach is to whip Benedick into challenging Claudio to a dual. (Which Benedick is not keen on because he’s war buddies with Claudio, but he plays it strategically by giving Claudio an opportunity to do the right thing while still challenging him.)
This is one of Shakespeare’s comedies, and so in the end everything works out after the twists and turns. Unfortunately, things even work out for Don John – for the time being, at least. The bastard (Shakespeare’s word) goes on the lam knowing the jig is up on his plot. His man, Borachio, rolls on him, and with Hero “dead” things are about to get serious. So, if you were expecting the villain to get it Shylock-style, you’ll be sadly disappointed.
Of course, it’s brilliant; it’s William -frickin- Shakespeare.