To understand how “The Descent” ends, we must start at the real inciting incident of the movie. Not our leads entering the cave, no. Sarah (Shauna Macdonald) loses her husband Paul (Oliver Milburn) and her daughter Jessica (Molly Kayll) in a car crash. Marshall shoots a pipe crashing through the windshield from Jessica’s POV, thus we see it impaling her father’s face head from behind before it keeps barreling towards the camera, and us.
One year later, Sarah’s estranged pal Juno (Natalie Mendoza) arranges the caving trip to bring their friend group of adrenaline junkies back together. Important addendum: Juno and Paul were having an affair, and this trip is Juno trying to face the combined guilt of that and abandoning Sarah while she was mourning.
Since this is a horror movie, the bonding of our leads — Sarah, Juno, Beth (Alex Reid), Rebecca (Saskia Mulder), Sam (MyAnna Buring), and Holly (Nora-Jane Noone) — is cut short by a cave-in. It turns out Juno brought them to an uncharted cave system, not the tourist trap they discussed, to spice up the adventure. As they dig deeper in hopes of finding an alternate exit, they discover why this cave remains undisturbed. It’s home to “Crawlers,” feral flesh-eaters who look like Nosferatu Neanderthals.
The group is splintered and picked off one by one, but Sarah and Juno become bonafide Crawler killers. Unfortunately, while fighting a Crawler, Juno accidentally strikes Beth with her pickaxe; a horrified Juno runs off, but not before Beth grabs her necklace (holding a pendant from Paul, inscribed with “Love Each Day.”) Sarah then finds the dying Beth, who tells her not to trust Juno (sealing the deal by showing her the pendant) and asks for a mercy killing.
Juno and Sarah, the last two left, both see themselves as murderers.