Why the third movie in a trilogy almost always sucks

• According to Rotten Tomatoes' rating of the 70 highest domestically grossing trilogies in the US, 64% of trilogies had their third film as their worst rated film.
• There are two factors that contribute to threequels almost always being the worst films of the series: the burden of having to wrap up the story and the fact that many threequels are produced solely to make more money.
• Threequels not only have to tell a great story that tops the films before it, but they also need to tie every loose end and somehow bring a satisfying conclusion to the saga.
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Following is a transcript of the video.

Narrator: There are certain rules when it comes to movies. It's always the girl who survives a killer, it has to rain during every funeral, and, yes, the third film in a trilogy almost always sucks.

And it's not just the terrible ones. Some of the most critically acclaimed trilogies also suffer from the threequel curse. And it got us wondering, what's really happening here?

The first thing we needed to find out was just how common bad threequels really are. And, surprisingly, the hardest part was deciding what exactly constitutes a film trilogy. In our case, it's a set of three films that share a continuous story, character, or theme. This means that independent trilogies like "Iron Man" and "Annabelle" within an existing franchise count, but trilogies that are continued years later like "Bourne" or "Pirates of the Caribbean" don't.

We then took 70 of the highest domestically grossing trilogies in the US and calculated each film's average Rotten Tomatoes score based on both critic and audience ratings to get a more accurate depiction. The list includes the obvious big hitters like "The Godfather" and "The Karate Kid," as well as a few unexpected gems like Ice Cube's "Barbershop" trilogy.

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Narrator: Average out all the scores for first, second, and third film, put them on a chart, and you get something like this. The first film was the obvious winner, with an average score of 71, followed by the sequel at 58, and threequels at 52. Count them all up, and 64% of trilogies had their threequel as their worst film.

So, why are threequels almost always the worst ones? There are two factors that come into play here: formula and necessity.

The reason why film trilogies have been around for so long is that they're a really great structure to tell a long story, with each film serving as a beginning, a middle, and an end. This also means that each film should serve a very specific purpose.

No matter the genre, the first film always serves as an origin story that introduces us to our heroes and the world they live in. That's why successful first movies usually fit into one of two categories. One, adapted materials from an existing body of work, like "The Silence of the Lambs" or the "Star Trek" reboot, that accurately capture the essence of the original source material, or, two, fresh, original IPs, like "The Matrix" and "Star Wars," that are quite unlike anything the audience has seen.

The sequels always tend to enjoy more freedom, since their characters and worlds are already established for them. For this reason, the second film is all about heroes facing the greatest challenge they've faced yet, whether it be iconic villains or a battle that just seems impossible to win.

This is where the problem with threequels comes in. There isn't really a formula for a successful threequel, at least not yet. And while sequels can usually stand on their own and focus on telling a good story...

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Narrator: Threequels have a lot more to accomplish. They not only have to tell a great story that tops the films before it, but they also need to tie every loose end and somehow bring a satisfying conclusion to the saga in almost exactly the same length as the other films. Studios often try to solve this problem with a bigger budget to visually raise the stakes. But it often just comes off as repetitive.

Some threequels actually repeat previous successes as a strategy, like how "Return of the Jedi" was really a rehash of "A New Hope" but with a bigger Death Star, and "Spider-Man 3," which crams so many villains into one film, just replicates the same plot beats as its previous film. Which brings us to an even bigger problem: necessity.

Trilogies suffer the most when they're just not needed at all. Skim through 33 of the worst-rated trilogies, and you could argue that about 85% of those films aren't needed after the first film. After "The Hangover Part II" destroyed everything the first film did right by replicating it, there were no loose ends that needed to be tied up, nor any separate conclusions to be made. But two years later, "The Hangover Part III" still arrived in the theaters with an even more abysmal rating.

So, despite all of this, why does Hollywood keep making terrible sequels? The answer seems obvious. Money, right? Well, the box office shows that each movie in a trilogy generally makes less money than the one that came before, but their differences aren't big enough to not warrant a threequel. On average, that third movie still makes four times more than its budget. And as long as we continue to watch them, the problem will likely never be resolved.

The curse of threequels is an issue that perhaps best illustrates the problems with how Hollywood approaches a film.

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Narrator: A good film can only be achieved with a good story. And once studios forget that eternal fact, what they're releasing at the end isn't a film, but a product without a soul.