Warning: Spoilers for "The Dark Tower" follow.

Fans of "The Dark Tower" already know the ambitious film adaptation will deviate from Stephen King's fantasy epic, but they likely don't realize just how significant the differences will be.

The author himself announced in March that the film doesn't center on his 1982 novel "The Gunslinger," but rather “starts in media res, in the middle of the story instead of at the beginning." Now Entertainment Weekly informs us that two major characters from the books, Eddie and Susannah, don't appear at all in the film.

However, if director Nikolaj Arcel's film is a success, and plans move ahead for the two sequels and companion television miniseries, they're guaranteed to be introduced. Wait, there's more: Arcel reveals "The Dark Tower" is actually a sequel to King's novels.

As EW explains, by the end of books, Roland Deschain has possession of the Horn of Eld, an artifact that "symbolizes a cosmic reset button." But in the movie, Idris Elba's Gunslinger already has the horn.

“The hardcore fans of 'The Dark Tower' series will know that this is actually a sequel to the books in a way,” Arcel tells the magazine. “It has a lot of the same elements, a lot of the same characters, but it is a different journey.”

These changes, and others, were done with King's blessing -- and, in some cases, his participation. "“I’m 100 percent behind it — which doesn’t mean it necessarily will work, just that it’s a good way to try and to get into these stories," the author says.

Opening Feb. 17, 2017, “The Dark Tower” also stars Matthew McConaughey, Abbey Lee, Katheryn Winnick, Tom Taylor, Fran Kranz, Claudia Kim and Jackie Earle Haley.