Disclaimer: In this three-act plot analysis, we will follow the plot of the Ring quest in this analysis, since that is the journey that frames the whole Lord of the Rings series. For more information on Aragorn & Company's plot line, check out Aragorn's "Character Analysis" or the "Chapter Summaries" of Book Three.
At the start of Book Four, Frodo and Sam have left the rest of the Fellowship of the Ring behind to travel directly to Mordor. But here's the thing: they are two tiny hobbits in the middle of the howling wilderness. How do they know how to get into Mordor? They really want to finish the task of destroying the Ring, but they need a guide… someone who is familiar with the countryside… someone who knows the secret paths surrounding Sauron's stronghold… someone like… Gollum? Yes, that's right, everyone's favorite slimy creature from The Hobbit has now become a major character in The Two Towers. With the addition of Gollum to Frodo and Sam's traveling party, they seem set on the right road to Mount Doom to accomplish their quest.
During Act II of The Two Towers, the plot's conflicts seem the furthest from their resolution. For us, that's Book Four, Chapter Six, when Gollum blames Frodo for his capture by the men of Gondor. The thing is, throughout Gollum's travels with Frodo and Sam, we have been wondering about Gollum's motivation. Gollum is a tricky little sneak, and it wouldn't be hard for him to think of some way out of his promises to help Frodo. But Gollum does seem to have some vague memory of earlier and better times, when he was a better, less dishonest creature named Sméagol. Maybe his good side will win out?
The suspense over Gollum's decision comes to a close in Chapter Six, when Gollum thinks Frodo has betrayed him to Faramir and the men of Gondor. Gollum is wrong, but his new resentment of Frodo tips the balance over to Gollum's evil side. It's at this point that we know that Gollum is going to do something bad to the two hobbits. We just don't know what yet.
We find out what exactly Gollum is planning at the end of Book Four, when Gollum tricks Frodo and Sam into entering the lair of the giant spider, Shelob. Shelob ambushes Frodo while Gollum leaps out of the dark and tries to strangle Sam. Sam fights off Gollum, but Frodo does not do so well against Shelob; she bites him and leaves him paralyzed.
Using the light of Eärendil and his sword, Sam injures or perhaps even kills Shelob. Now, Sam and Frodo have indeed made it into Mordor, but Frodo has been poisoned by Shelob and taken prisoner by a party of orcs, leaving Sam alone and helpless in the western mountains of Mordor. So the Two Towers plot arc ends with Frodo and Sam closer to their goal but still stuck between a rock and a hard place. There are plenty of loose narrative threads for Tolkien to pick up in The Return of the King.