Note: Spoilers alert!
It is set in Middle-earth, a fictional world. The main protagonist is Hobbit Frodo Baggin who is on a quest alongside his friends in order to destroy the One Ring. Gandalf the Grey, a wizard is the one who assigns him the task. As Frodo goes out on the quest, Gandalf starts uniting the people of Middle-Earth against the Sauron's forces.
The middle-earth has everything, from dwarfs to elves. From Gaint eagles to scary dragons. Amazing scenery, and cinematography. Huge wars between the Good and Evil forces. Romance, brotherhood and many other things.
In The Fellowship of the Ring, Bilbo who founds the ring after it was lost for a long time celebrates his 111th birthday. He then tells Gandalf that he wants to leave the shire, and give his inheritance, including the One Ring to his nephew Frodo. Gandalf then tell Frodo to go and destroy the ring. Frodo goes on with his friend on the quest.
In The Two Towers, there is a big war that is happening in Helms Deep after Uruk-Hai of the Sauron Forces try to destroy the Rohan. They evacuate the people to Helms Deep which is an ancient fortress which managed to defend against every kind of attack. Although this impregnable fortress has one weakness, and it's the sewer system. After a deadly battle, when Rohan are on the brink of defeat. Gandalf along with Rohirrim, Fierce battalion of Horse riders arrive to help, and they win the battle.
In The Return of the King, There is a battle in Gondor, where the Sauron forces arrive with all their force. The Gondor forces are less in number, and they are almost defeated. But before the war Gandalf light the fire torches which give a signal to the Rohan, who comes out in order to help the Gondor against the Sauron's army. Later on, Frodo manages to destroy the ring, and then everyone lives happily after.
No matter how many time you watch this masterpiece, it never gets old.