When Simba returns, he finds Pride Rock in a mess because of Scars improper rule over the land. Even Zazu is caged, and the hyenas are allowed to freely roam the Pride lands. In order for Simba to get to Scar, his friends had to take care of all the hyenas.
Scar accuses Simba of murdering his father, and then attacks him.
Scar attacks Simba, and Simba doesn't fight back because he does not want to kill his own uncle.
Simba gets into truble.
Scar sees Simba in almost the same exact position that Mufasa was in when Scar killed Mufasa. So Scar decides to take advantage of that. But Scar tells Simba that he actually was the one that killed Mufasa by this same exact means.
Simba, now realizing that it wasn't his fault that Mufasa died, gets up enough stregth to challenge Scar and defete him.
Simba remembers all that Mufasa had taught him, and takes his rightful place as king of Pride Rock.
And peace settles over Pride Rock once again with the proper king on the throne.
And once again, the circle is completed and begins once again.