The Kingdom of the Crystal Skull is considered by most fans to be the worst Indiana Jones movie, and based on comments from former Lucasfilm boss Kathleen Kennedy, we now know why it turned out the way it did.

Warning! Spoilers for Indiana Jones and the Kingdom of the Crystal Skull follow.

Indiana Jones and the Kingdom of the Crystal Skull, directed by Steven Spielberg with a story worked on by Star Wars creator George Lucas, sees Harrison Ford reprise his role as the iconic adventuring archeologist in a search for a telepathic crystal skull located in Peru in 1957. Helped by Marion Ravenwood (Karen Allen), and their son, Mutt Williams (Shia LaBeouf), Jones eventually witnesses otherworldly beings leave our dimension in what very much looks like a flying saucer, giving the impression it was about aliens all along.

In a feature on Spielberg and his movies by Vulture (paywall), Kennedy and Lucas discuss the difficulties they and others faced making the movie. First up, cinematographer Janusz Kamiński admitted he failed to recreate the look of the previous Indiana Jones movies established by Douglas Slocombe, calling The Kingdom of the Crystal Skull "by far was the hardest movie I’ve ever done in my life."

Kennedy said The Kingdom of the Crystal Skull was a tough production not just for Kamiński, but for Spielberg and Ford, who she said were "struggling" with it because they didn’t want to do a "Raiders movie that involved aliens," and "kind of got into a fight" with George Lucas about it.

Lucas told Vulture: “I wanted it to be kind of a War of the Worlds sort of thing. Harrison said, ‘I’m not going to do another science-fiction movie.’ And Steven said, ‘I’m not going to do another science-fiction movie.’ I said, ‘Steven, this is perfect because it’s the 1950s, when flying saucers were a whole thing,’ but he said ‘no.’ We did about five scripts, and finally Steve and I compromised: ‘Look, what if they’re not aliens but from another dimension.’”

Kennedy said Spielberg and Ford went along with Lucas’ idea, but they “were not 100% onboard.” And that’s the root cause of The Kingdom of the Crystal Skull ending up as the worst of Spielberg’s Indiana Jones movies.

“That’s why the movie, out of the four that Steven made, is the weakest,” Kennedy said. “And that’s why Harrison was so deeply committed to Destiny. He didn’t want that to be the end.”

Lucas went on to discuss the final, divisive shot of Indiana Jones and the Kingdom of the Crystal Skull, where the interdimensional beings fly away in an alien-like craft. “Steven put that last shot in, where they get into a flying saucer and take off,” Lucas said. “He was rationalizing it by saying, ‘Well, they’re going to another dimension. They have to get there somehow.’ I said, ‘It looks like a flying saucer.’ He did make a science-fiction movie after that, and Harrison did an alien movie.”

After Indiana Jones and the Kingdom of the Crystal Skull came out in 2008, most thought that was it for Ford playing the role, but he returned once again for yet another entry, 2023’s Indiana Jones and the Dial of Destiny. Here, Kennedy is suggesting here the way The Kingdom of the Crystal Skull went pushed Ford to want to do The Dial of Destiny.

Earlier this year, Kennedy indicated it may be some time before we see another Indiana Jones movie, insisting: “I don’t think anybody is interested right now in exploring it.” Three years on from The Dial of Destiny, Lucasfilm has yet to indicate that a follow-up is in the works.

Wesley is Director, News at IGN. Find him on Twitter at @wyp100. You can reach Wesley at wesley_yinpoole@ign.com or confidentially at wyp100@proton.me.