Richard Brinsley Sheridan's School for Scandal is a play that results entirely from, well, scandal. For a period of time in the eighteenth century, rumor became a huge part of high society. Everyone wanted to know what everyone else was doing, even if they weren't actually doing it. Newspapers became a huge vessel of this scandalmongering, but in Sheridan's play, we see the spread of scandal—and its consequences—on a much more individualized basis.
There are a lot of characters, but the three that are the most important to distinguish (and try to keep straight) are Joseph Surface, Charles Surface, and Maria.
Joseph is considered by most to be the more virtuous of the Surface brothers. He desires Maria's hand in marriage, and just about everyone agrees that this is a smart match—everyone except for Maria, that is. Maria wants to be with Charles, but Charles is a party boy, and since Maria is the ward of a man named Sir Peter, she basically needs Sir Peter's permission, which he won't give.
This is, on a very basic level, what the play attempts to resolve. However, it's a lot more complicated than that, which brings us back to scandal.
An unpleasant woman named Lady Sneerwell has paired up with Joseph to start a rumor that Charles is having an affair with Sir Peter's wife, Lady Teazle, in the hopes that Maria will find out and think that Charles is a bad man. It's incredibly important to note, here, that Maria is one of the few characters who absolutely despises gossip—so Joseph better hope she doesn't find out he's spreading lies!
Amidst all of the lies and trickery that go on throughout the first three acts, Sir Oliver, uncle of the Surface brothers, returns to England for the first time in over a decade. For some reason, he suspects that Joseph is not as admirable as everyone thinks he is, and also has a sneaking suspicion that Charles isn't such a bad guy. So what does he do? Goes undercover, of course, to try to catch the brothers off guard.
He comes to find that Charles, though most definitely a party boy, admires him (Sir Oliver) in a way that absolutely charms him; Charles is ready to sell just about everything he's inherited from the Surface family to gain a little credit, but he refuses to sell Sir Oliver's portrait. Sir Oliver's ego is quite flattered by this, apparently.
Joseph, on the other hand, lies straight to Sir Oliver's (disguised) face, saying that his uncle never sends him much money and any money he has goes straight to Charles-the-money-waster. The catch is that Sir Oliver has just recently sent him a pretty fat check, and there's no way that money is already gone. Joseph is selfish and he is stingy.
If it's not clear already, all of Joseph's lying is going to bite him in the behind, which will ultimately resolve the plot. By the end, everything comes out and Joseph metaphorically falls from grace. Sir Oliver convinces Sir Peter of Charles's virtuousness, everyone is convinced of Joseph's manipulative and selfish ways, and Maria hates Joseph more than ever. Naturally, Sir Peter decides that she should, in fact, marry Charles.
Of course, Sheridan gives us a (sort of) happy ending, but he also leaves us with a message: don't lie and don't spread rumors, because all you're causing is damage.