The following is a very brief summary of the book, taken from BookRags Notes. They also have a complete, chapter-by-chapter summary.

Gandalf and Pippin ride away from Merry and Aragorn and go to Minas Tirith. Here they share their tale with the Steward of the city, Denethor, and they speak of his two sons Boromir and Faramir. Pippin pledges himself to the Steward and offers his services. Aragorn’s kin join him and tell him that he should follow the paths of the dead. Aragorn leaves with Legolas and Gimli and refuses to take Merry or Eowyn with him. Theoden rallies the men of Rohan to go to war. He tells Merry that he cannot come with him. Merry is very upset, but a quiet rider tells him that he will secretly bear him to war.

Pippin speaks to Denethor again and then begins his duty as a guard of the city. With another guard, they watch bands of warriors file into the city from every region of Gondor. The number is too small. Faramir returns but is sent by his father to defend another part of Gondor. The enemy comes and besieges the city. Faramir is badly wounded and Denethor retires to a chamber of the dead to end the lives of his son and himself. Pippin gets Gandalf to stop him. Just as the attacking army breaches the door of the city, the riders of Rohan appear and attack with voracity. The Lord of the Nazgul kills Theoden. Merry wounds the Lord and Eowyn kills him. Both fall terribly ill because of this attack. Aragorn arrives from the south with reinforcements and the armies of Mordor are repelled. He heals Faramir, Merry and Eowyn and then makes plans to leave for Mordor with an attacking force. They march to the gates of Mordor and demand Sauron’s surrender, but the Lord of the Dark refuses. He unleashes his trap and they are embroiled in a giant battle.

Sam is tempted by the ring but he resists it and returns to Minas Ithil. He finds Frodo and he frees his friend from the torture of the orcs. They begin to make their way slowly into Mordor. The land is desolate. There is no water to be found. With each step it becomes more hopeless. They are mistaken as orcs and forced to march. When they slip away they come to the edge of the mountain. Sam carries Frodo part of the way up. They encounter Gollum again and Sam tries to contain him as Frodo goes on the last step of the journey. Frodo is taken over by the power of the ring and it is destroyed only because Gollum bites it off his finger and falls into the pit of doom. When the ring is destroyed Sauron dies and the armies of Mordor fall apart. Gondor is triumphant. Gandalf takes three great eagles and flies to Mount Doom. There he finds Frodo and Sam awaiting their deaths. He rescues them and takes them to Ithilien where Aragorn and the armies honor them as heroes.

They return to Minas Tirith and Aragorn is crowned king. They linger here until he is wed to his love Arwen. They all travel to Rohan and have a funeral feast for Theoden. The marriage of Faramir and Eowyn is announced. They head towards Rivendell. Frodo is reunited with his uncle Bilbo. From here they travel back toward the Shire, and Gandalf leaves them. Merry, Pippin, Frodo and Sam, the four original companions, return to find their home overrun by large men led by a man named Sharkey. They raise the hobbits around them and defeat the men in a battle. When they find Sharkey, they find the fallen wizard Saruman who gets killed by his own servant. Sam gets married and they rebuild the Shire. Frodo gets increasingly weaker as the years pass. Eventually he decides to leave and meets Bilbo and some elves on the road. They take him with them across the sea where elves go when they tire of the earth. Sam returns home to his wife and children.