Gandalf and Pippin arrive in Minas Tirith, capital of Gondor. Pippin joins the small band of soldiers who defend the city against Sauron's armies.
Aragorn leaves to travel "the paths of the dead" to command the ghosts who are there to fight for him. The men of Rohan ride to Minas Tirith, but Merry is not allowed to fight. They reach Rohan just as its gates fall.
Merry and Theoden's daughter Eowyn kill the King of the Black Riders. Aragorn arrives with people he freed using the ghosts he got earlier (dead complicated, I know) and the battle is won.
The remaining armies ride to Mordor, trying to draw Sauron's attention away from Frodo and Sam. But Frodo's things are shown to them, and they believe that everything is lost...