The alternate storyline of this campaign is divided into more distinct books than in the original. The first book, The Fellowship of the Ring, is mainly unchanged. However, that is the end of the main Frodo storyline. Frodo appears as a minor character in the later books as he and his friends travels through Rohan to find his way back home, but they are a sideline.
This is the story of how Frodo and his fellow hobbits brought the One Ring across the land and through the Mines of Moria to Lothlórien. They braved dangers in the Old Forest, eluded the Nazgûl, and recovered in Rivendell. There they formed the Fellowship of the Ring, and brought the Ring through the mines at the expense of Gandalf the Wizard's life. They went on to meet Galadriel, whom Frodo gave the One Ring to. The Fellowship are surprised at his decision, but come to accept it.
This story departs from the One Ring to follow the fate of the Fellowship. They agreed to split into two group leaving Lothlórien. Aragorn and Boromir would go to Gondor accompanied by Galathil (brother of Celeborn) and his men. Legolas and Gimli would go with the hobbits to Rohan, accompanied by Haldir and his men. They were ambushed by Saruman's Uruk-hai by the Falls of Rauros, but defeated them.
The first party found a rough welcome at King Theoden's hall in Edoras. With help of Eomer, they cast out Wormtongue and restored the King and rode with him to battle. With the help of Haldir's men, they defeated Saruman's forces at Helm's Deep. Proceeding to Isengard, they were amazed to find Gandalf resurrected and Saruman defeated with the help of the Ents.
The second party also found a rough welcome with Lord Denethor, who became very hostile upon learning Aragorn's identity. Boromir at first sided with his father and plotted against Aragorn, still seeking to gain the One Ring for Gondor. However, he recovered and made up with Aragorn. Aragorn then left to treat with Rohan while Boromir lead the fight against Mordor with his brother Faramir. Denethor was outraged that Boromir would defy him, and died under mysterious circumstances.
Lead by King Boromir and Queen Eowyn, the combined forces of Rohan and Gondor cross the Anduin and besiege Minas Morgul.
At Erebor the Lonely Mountain to the North, Galadriel's emissary Orophin arrives just as the forces of Mordor are gathering to besiege it -- bearing a King's ransom of gifts. He stays and advises, and after some debate King Dain II decides to force a break in the siege. He and Lord Brand of Dale (son of Bard) fight the besiegers and emerge victorious, but at great cost. During this time, Rombur son of Bombur has excavated in the depths of Erebor and found secret chambers near the tomb of Thrain I who founded Erebor after the loss of Khazad-dum, with the help of his friend Nurri. He works on translating the runic secrets there, but his friend Nurri secretly stole a dragons-tooth horn from the tomb of Thrain I.