In 47 Meters Down (now on Netflix), Mandy Moore and Claire Holt star as vacationing sisters whose shark-watching day-trip turns into a living nightmare. Once the winch break, they are plummeted to the gulf’s bottom, trapped in a cage, running out of air, and surrounded by sharks. Johannes Roberts’s deep sea-set thriller precisely steps through the resilient pair’s quest to survive, following their furtive swims outside the cage to make radio contact to could-be rescuers, and to retrieve a survival gear. But in its final act, this shark tale takes a trippy turn that’s got people talking. So, what’s the deal with 47 Meters Down’s ending?

Spoilers below. Like major spoilers that will spoil the end of the movie. So if you haven’t seen it, check it out on Netflix first.

Kate (Holt) and Lisa (Moore)’s scuba tanks are critically low on air. The back-up winch also busted, leaving their hopes battered, and Lisa’s right leg trapped under the freshly fallen cage. Desperate, Kate swims out of the cage once more so she can radio Captain Taylor (Matthew Modine) for assistance. “My gage is on 5-bar,” she gasps, “(My tank is) going to run out any minute.”

Taylor tells her he’ll send down extra air tanks, and that the Coast Guard is on the way. Because Lisa’s leg is pinned under the cage, Kate must be the one to venture out and retrieve the tanks. With her tank nearly empty, she connects to the second with seconds to spare. Her rush of relief is short-lived. Out of sight of the cage, Kate radios Lisa, telling her she’s got three flares and the tanks. And all they need to do is wait for the Coast Guard. But before Kate can return to the safety of the cage, a shark snatches her and drags her out of sight.

Lisa cries out for her sister to no avail. Kate is gone, dead or at least out of radio range. Kate was the one who knew about scuba diving. Without her to rely on, Lisa realizes she has to figure out how to retrieve to the dropped tank, or else suffocate before the Coast Guard arrives. Using the spear gun as a crude grappling hook, she tugs the tank to herself, but not before slicing open her palm on the spear. In the nick of time, Lisa hooks up the new tank, and for an unknown period sits, slowly breathing in the fresh tank’s air. Then, comes a new rush of hope as she hears Kate calling to her over the radio. Kate says she’s wounded and surrounded by sharks. Lisa is determined to rescue her.

Using the lesson in BCD (buoyancy compensation device) that Kate taught her earlier, Lisa uses the device to lift the cage just enough to wriggle her leg free. From there, she uses the radio and flashlight to locate Kate. Together, they must swim to the surface in shark-infested open water for more than 150 feet. But to avoid the potentially fatal affects of the bends, they’ll have to take a “decompression stop” for five minutes, meaning five minutes of dodging sharks while treading water below the surface. It’s an absolutely terrifying proposition, but the formerly scared senseless Lisa turns action-star hero, one arm around her wounded sister’s waist, the other wielding a flare to keep the sharks at bay.

Against all odds, the sisters make it to the surface, but before reaching the boat, Lisa’s right leg is grabbed by a shark. She attacks the shark’s eye to break free of its powerful jaws, and is finally pulled to safety. Aboard the run-down boat she’s reunited with Kate. But as she sighs with relief and joy, something strange happens. Blood drifts from her wounded hand and into the air. As he leans over her, Taylor’s voice seems strange, like it’s coming through a squawky radio. Then 47 Meters Down reveals the terrible truth: Lisa is still in the water, still in the cage, talking to herself as the Coast Guard finally comes to her rescue.

So what the hell?

The key to this sequence is something Taylor told Kate in their last communication:

“I’m going to send down extra air tanks. But Kate, there’s a reason that I didn’t do this before. Moving onto a second tank is going to increase your chance of nitrogen narcosis dramatically. You’re going to need to watch each other closely to make sure that neither of you is behaving strangely or hallucinating.”

Everything that happens after Lisa connected her second tank up until the Coast Guard swims up to her cage is a hallucination. Alone at the bottom of the ocean, her brain gave her a hopeful hallucination to help her survive. In her own mind, Lisa became an action hero who got to nobly save her beloved sister. But in a way, it was Kate’s final rescue attempt, as the hallucination folds in Kate’s mentioned three flares, her bends warning, and the BCD lesson from earlier. We never see Kate warn Lisa about nitrogen narcosis. Which makes sense, as she’s trying desperately to keep her big sister from a breakdown. And Lisa’s memory of Kate preserves her in those final minutes before help arrives.

The movie gives warnings this action-packed rescue sequence isn’t “real.” For one thing, the leg that the shark bites is the same one that’s been trapped under the cage. It’s as if the pain of that pressure is informing Lisa’s hallucination, seeking a reason for the hurt. The subtler clue is that this movie that doggedly strived for a chilling authenticity in its action and staging abruptly turns to the outlandish plan to out-swim sharks and chase off a feeding frenzy with a single flare. Also, note that her hallucination begins and ends with the same image: the blood from her hand drifting, as if in water.

In the reality of 47 Meters Down’s ending, Kate is shark bait. She’s dead. Lisa survives. But at least for a moment, Lisa got the happy ending we wished for her and her sister.

Kristy Puchko is the film editor of Pajiba. You can follow her on Twitter.