The weakness of trilogies (also, pirates!)

I want to talk about trilogies. And I'm going to use the Pirates of the Caribbean trilogy as my example, because there's a good chance that most of you've seen those movies.

Yes, I'm ignoring that On Stranger Tides even exists, because it's different.

Now, everyone can probably all agree that the first movie, The Curse of the Black Pearl, was awesome. We were introduced to a new world with drunken pirates and people that could become undead skeletons without dying. No one expected that movie to be what it was, and that's one of the things that made it great. Not to mention Jack Sparrow.

Let's skip ahead a bit. The third movie, At World's End, was also pretty exciting. All of the quirkiness and drama in the previous Pirates of the Caribbean movies was amplified and brought the trilogy to a satisfactory conclusion. There was also an epic pirate battle and a giant crab woman. It's kind of difficult to beat that.

So you might be wondering why I skipped the second movie. First, let me be clear: Dead Man's Chest wasn't a bad movie. There was a kraken, and you guys know how I feel about squid. The things is, I don't think Dead Man's Chest was as good of a movie as the other two.

This isn't an uncommon thing. In trilogies one of the installments always seems to suffer and it's usually the second. Why? Because of exposition. Think about it--most of Dead Man's Chest was spent on exposition. We learned about a lot of crazy new stuff, like Davy Jones and Will's dad. Things were further complicated by the love triangle and the British navy and Jack's schemes. But none of the loose ends were tied up. Instead they were all saved for At World's End.

Which is probably a good thing, because if Dead Man's Chest hadn't been the weakest link, then At World's End would have suffered as a movie. And you can't end a trilogy on a dull note.

So what do you think? Is it just in the nature of a trilogy to have a not-so-great second installment? Is there any way to keep a three-part storyline from sagging, or is it inevitable? And why do you think trilogies suffer from this issue, but not extended series like Harry Potter? Tell me your thoughts in the comments.