“Should we be running?” asks Jessica Chastain in “It Chapter Two.”

Um, yeah. You’re in a very gory sequel to a very gory horror movie. You should be running instead of: dropping in on a spooky, abandoned theater. Sticking a hand down the sewer drain where your baby brother was snatched by a monster. Or paying a visit to the decrepit childhood home where you were repeatedly assaulted. And, yet, the characters in “It Chapter Two” — who are supposed to be veterans of Stephen King-style childhood horrors — do those dumb things and more.

“Chapter Two” picks up 27 years after the events of “It,” in which seven adolescent friends were terrorized by the evil clown Pennywise, who fed on their secret fears. Ultimately, they vanquished him, but they’ve gone their separate ways, blanking out their memories of each other, until a friend summons them home to take on Pennywise again.

The setup is potent because King is so good at exploiting primal emotions. Much more than a horror movie, “Chapter Two” is an unsettling stew of grief, insecurity and guilt — not exactly the tastiest of stews, but one we can all relate to. Some of the new encounters with Pennywise are well executed by director Andy Muschietti, who also made the first “It.” That one relied too much on shocks, rather than scares, but “Chapter Two” creates real suspense in, for instance, a claustrophobic encounter in a mirrored funhouse.

Essentially, the new movie covers the second half of King’s 1,138-page doorstop, but even in biting off only half of the book, there’s too much material. “Chapter Two” languishes over two hours and 49 minutes and it feels even longer because of its structure. “Chapter Two” often has the friends — played as adults by Chastain, Bill Hader and, disconcertingly, Old Spice pitchman Isaiah Mustafa — split up to pursue individual nightmares, before reuniting to battle Pennywise together. That feels very like an old-style video game, rather than a narrative. Instead of building and building, the movie keeps resetting at zero as a new character takes on a new “level.”

A bigger issue is that one-third of “Chapter Two” consists of flashbacks to the time frame of “It,” with the kids from that movie doing battle in new situations. Muschietti clearly had a much bigger special effects budget this time out, but it’s hard to care about the fates of the kids in those scenes because we know for sure that they’re going to survive. Not only survive, but grow into an Oscar-nominated actor, an Emmy-winning sketch comic genius and a guy who apparently smells really good all the time. As a result, the flashbacks feel like they’re marking time.

Hader is the MVP of “Chapter Two” (along with King, who pops up in a polished cameo), and he gets to do some of his most unexpected work toward the end, when the stakes are finally raised as the team battles Pennywise. The last 45 minutes of “It” feel like a completely different movie, veering toward sci-fi fantasy, and Muschietti has trouble controlling the action, which seems to have about five different finales. In the end, that’s the legacy of “Chapter Two”: There’s some good stuff here, but they had no idea how to end “It.”