“Star Wars: The Empire Strikes Back” celebrated its 40th anniversary Thursday.

The film — the second one to be released and the fifth overall in the saga — tells the story of Luke Skywalker learning how to become a Jedi with the help of Yoda. Meanwhile, Han Solo and Leia evade the Empire, rushing to Cloud City to meet with Lando Calrissian.

It’s often considered the best “Star Wars” film by fans. It has the highest score on Rotten Tomatoes with 94% (“A New Hope” is right behind with 92%).

That’s partially because the film contains one of the biggest surprises in movie history. Spoilers if you haven’t heard it already. Darth Vader reveals he’s Luke Skywalker’s father.

Would the twist stayed hidden in the age of the internet, though? “Star Wars” creator George Lucas said it might have been kept secret, according to an interview with StarWars.com.

“I think it might have!” he says. “Because the thing about it is I didn’t tell anybody — anybody — about it. And it wasn’t in any of the scripts. It wasn’t even in the story treatments. I kept that aspect of it secret and I was the only one that knew about it. And it really wasn’t until the day we shot that we told Mark (Hamill) so he could react appropriately.”

But, then again, maybe the internet would find out the truth.

“I think, in this era now with the internet the way it is, it’s very hard to have surprises in a movie. And I don’t think you could do it today.”

Thankfully, Lucas didn’t have to worry about the secret.

The list of people who knew about Darth Vader was limited. Mark Hamill, who played Luke Skywalker, was the third person to learn about it, aside from director Irvin Kershner and George Lucas. James Earl Jones later learned the truth when recording voice-over work for Vader. Sound booth mixers and editors learned about it, too.

The original script read, “You know the truth. Obi-Wan killed your father.” So that’s what David Prowse, who was under the Vader mask, said to Hamill when they acted out the scene, according to ScreenRant.

The cast didn’t learn the truth until after the film premiere.