Through means never fully explained (unless you buy the comics and video games), the ring getting destroyed was all part of Sauron's master plan. The villain for the first two sequels is Samwise's son Bobbit, who Sam tried to kill when he learned he was dabbling in dark arts. He's redeemed by our hero, an orc that broke rank with Sauron after learning about his plan to, IDK, blow up the whole world or something, but he's killed anyway to make room for our third movie reveal that Sauron was behind it all. Our brave orc hero must meet up with a haggard looking Aragorn, Gimli and Legolas to painstakingly re-enact the events of the Fellowship of the Ring, wander around with Gandalf and learn how to become a wizard, then kill Sauron and take the last name Baggins.