One does not simply give a brief summary of the extended edition of The Lord of the Rings: The Two Towers… but we'll try. To aid us in this venture, let's look at each of our three journeys one at a time, starting with Merry and Pippin.
Held captive by Saruman's orcs, our two hobbits are headed by to Isengard because they think they've found the Ring bearers. After almost being eaten, the orcs are attacked by Éomer's company of Rohirrim riders, allowing Merry and Pippin to escape their bonds and head into the treacheries of Fangorn Forest. Again, they're almost eaten by an Orc pursuer, but Treebeard, an ent (or large, humanoid tree) squashes him and grabs Merry and Pippin, taking them to the White Wizard to determine whether they are orcs or not—uh oh. Wait, it's okay. The White Wizard is actually Gandalf who, after falling into the depths of Moria, defeated the Balrog, and came back stronger and shinier than before.
Turns out, Merry and Pippin are not orcs (phew, we were worried), so Treebeard takes them to his home where they'll get to sleep through an ent council during which the ents very slowly decide, again, that Merry and Pippin are not orcs (you can never be too sure, those tricksy hobbitses). Then they decide not to go to war against Saruman. Pippin, thinking on his feet—or on Treebeard's back anyway—tells Treebeard to carry them south, toward Isengard. He obliges and upon reaching the edge of the forest, sees the massive amount of his brother trees that have been burned to the ground. Enraged, the ents assault Isengard, breaking the dam and bringing Saruman's domain to ruin.
Our second storyline follows the journeys of Aragorn, Gimli, and Legolas, who begin by tracking the orcs carrying Merry and Pippin so they can save their friends. They follow them into Fangorn where they instead meet Gandalf. Gandalf assures them that the hobbits are safe, and together the four head toward the Rohan capital of Edoras, where they overthrow Saruman's hold of King Théoden and banish the wicked Wormtongue.
We learn that Saruman has been brewing up quite the orc army in Isengard, and seeks the destruction of Rohan. The newly livened Théoden orders his people to retreat into the fortress Helm's Deep, where they might stand a chance against the orc hordes. On the way, they're attacked by warg riders and Aragorn falls off a cliff (but don't worry, he's okay).
Saruman's orcs march on Helm's Deep and even with the elf reinforcements that show up, Saruman's magical black powder is enough to destroy the wall and turn the tides in favor of the White Hand. The keep is all but taken when Aragorn convinces Théoden to ride out one last time. As they do, Gandalf appears with Théoden's nephew, Éomer, and the Rohirrim riders. They converge on the orcs and the battle is won.
Finally, we can't forget about Sam and Frodo. Aware that Gollum is following them through the rocky Emyn Muil, they capture him and drag him along until he promises to lead them toward Mordor. Sam doesn't trust him but Frodo has pity on him and he does, in fact, lead them out of Emyn Muil, through the Dead Marsh, and to the enormous Black Gate of Mordor. But when they get there he can't let them go. Too afraid they'll be caught, he tells them there is another way, so they divert their path—only to be captured by Faramir and his Gondorian soldiers.
Gollum was able to evade capture but they find him later, fishing in a sacred pool. Frodo is able to save his life by coaxing him out, but when the men bag and beat Gollum, all trust he had in Frodo is gone. Faramir takes his captives to Osgiliath where he plans on handing them to his father, the steward of Gondor. But after hearing of Boromir's fate and seeing how the Ring affects Frodo (the Ring's strength grows as Frodo's willpower fails), he decides against it, letting them go. As they travel back to Mordor, we hear Gollum making plans for "her" to kill the hobbits so he can take back his precious. With an ominous shot of Mordor, the movie ends.