In a remote area of Texas, archaeology professor Hopper and his dog are looking for a group of
hippies that went missing in the 70s. He finds a van that was abandoned there decades ago with a
journal, which has a picture of the owners and notes with drawings about the local legends.
Nearby, he sees a deep hole in the ground that his dog won’t come close to. There’s also a cave up
in the hill, so Hopper goes inside to investigate while his dog won’t stop barking at it. Using his
flashlight, Hopper sees an armed cowboy standing perfectly still inside the cave and his curiosity
increases. He immediately returns to town and asks his students Jackie and Taylor to get him
a climbing kit ready, but he also announces that they won’t be coming with him like they originally
planned. Moments later, Hopper returns to the same area and just in case, he ties a rope to a rock
and throws it into the hole. Then he goes back to the cave, which his dog refuses to enter. Once
inside, Hopper sees the same cowboy standing in exactly the same spot as last time and carefully
approaches him. Suddenly he feels something like an invisible shimmer in the air and touching
it reveals moisture, so he thinks it’s like a transparent wall and crosses it anyway. Two days
later, Taylor tells Jackie they should go looking for their professor, who has been missing since
he left. Jackie refuses to go hiking because it’s dangerous, but she accepts to go if Taylor gets
them a vehicle. Taylor has no choice but to call their classmate Cara, who has a crush on him. Cara
brings her dad’s jeep but also her little sister Veeves and the girl’s best friend Furby, who has a
crush on Jackie. The kids will record everything for a school project and their social media,
and they’ve brought cameras for everyone. On their way to the cave, Taylor explains Hopper has been
obsessed with finding a group of hippies that got lost in the 70s while looking for the Fountain of
Youth, which supposedly is hidden in the area. When they arrive, the group checks out the van
and when they find the pictures, they discover the missing hippies were actually Hopper’s parents,
which explains his obsession. After they take a group photo, Furby trips over some worn-out
rope tied to the ban, so they follow it until they find another cave entrance. Ignoring the
spiders as they go deeper inside, Jackie goes down through a small tunnel and discovers that the rope
has been cut. Taylor takes a closer look at it and decides it’s not Hopper’s rope because it’s
too old. Jackie looks around the tunnel with her flashlight and notices that the cave is very deep,
so they’ll need proper equipment to go looking for Hopper safely. The group gathers all the
exploring equipment from the jeep and since Furby’s staying because he’s afraid of the
dark and creepy crawlers, they leave him a radio so he can keep watch. Very carefully, the group
starts climbing down the tunnel in the cave and they come across the mysterious shimmer, which
they touch and notice how the air changes. Then a flashback reveals what happened to Hopper when
he crossed the barrier. As he went deeper inside, the cowboy started to move as well and a strange
light began to flicker. Suddenly Hopper heard some creepy noises and a scream, so he immediately ran
away. Once outside, he was shocked to see it was already night time even though he had been in the
cave for just a few minutes. His dog was nowhere in sight and when he reached his truck, he noticed
it was covered by overgrown plants and his ID was worn out. Hopper decided to search the area,
which was covered by overgrown plants too, and found a jeep with a bag indicating this belonged
to his students. It seemed the cave had sent him into the future. Worried about his students,
Hopper went back into the cave, leaving a trail of lights in the process. One of those lights
suddenly started floating under the influence of the transparent barrier, so Hopper kept
going. Back in the present, the group continues to explore the cave and in another dark tunnel, they
hear the same creepy noises Hopper heard before. The girls are scared of something dangerous living
here and decide to leave, ignoring Taylor’s worry over Hopper. Jackie goes first but as she begins
climbing, the rope is suddenly cut off, causing her to fall. Taylor tries to catch her and fails,
so he hurts his arm and Jackie hurts her leg. Then Taylor tries to check the second rope,
but that one has been cut off too. Some of them think Furby did it as a prank although Veeve says
he isn’t the type to do this, but when they try to contact him through the radio, he doesn’t answer.
The group starts to argue over what to do next only to be interrupted by a loud thud coming from
the tunnel. Taylor tries using the radio again and this time a raspy voice replies ‘help me’
and says his name is Furby. The group isn’t sure if believing him because the voice doesn’t sound
right and an argument ensues which only stops when they hear another noise coming from the tunnel.
Taylor, Veeves, and Cara decide to look for Furby in the tunnel while Jackie stays behind because of
her leg. She tries to follow what they’re filming with her phone, but soon the signal is lost. After
walking for a while, the group reaches a spot where the sky is visible and they notice the
light keeps flickering. Cara climbs on a rock and finds a broken rope, and when she looks behind it,
she finds something that makes her cry. She begs Taylor to take Veeves into the other room, but at
that moment Jackie arrives crawling and they use a camera to spy what’s going on: to their horror,
it’s Furby’s dead body. Taylor points out that Furby’s neck is broken and there’s more rope
on the ground, meaning Furby had tried to climb down and someone cut his rope. He also discovers
Furby’s camera is still working, so they decide to watch the whole recording to see what happened. In
the video, Furby complained about losing contact with the group because nobody answered the radio
or his yelling, and to their shock, days started to pass while the boy waited for them outside. His
phone ran out of battery, so when he was bored, he read the journal from the van. The story said
a bunch of Conquistadores were sent by the Queen to find a fountain with magical properties that
could heal people or even bring them back to life. Hopper’s parents went looking for it when Hopper’s
sister got sick with cancer. Days continued to pass outside and eventually Furby ran out of
food and water, so he wanted to leave. He thought his friends were dead, but he still needed to find
them to get the jeep keys. Furby began climbing down the hole and when he crossed the invisible
barrier, he noticed the sky was changing at crazy speed. Suddenly his rope snapped, and poor Furby
fell to his death, explaining the thud the group heard earlier. Shocked and disturbed, the group
stops the video and discusses what they saw. It seems days passed outside while they’ve only been
here for an hour, so something is going and they need to leave immediately, especially since Taylor
thinks that the flickering light is the sun going through the day-night cycle very fast. Since Cara
is the only adult with no injuries, she will be the one to climb out and get help by using a SOS
beacon. As she starts going up, Cara can feel the air changing, and soon she makes it out only to be
shocked by the landscape she finds: the whole area has become a creepy barren land. To make
matters worse, the rope is gone and Cara can’t stop coughing because of the weird air. As a huge
sandstorm starts approaching in the distance, Cara tries to use the beacon, but there’s no signal. At
that moment, Cara sees a strange triangular object floating in the sky and decides to go back down.
When she reunites with the group, she tells them what she saw but they don’t believe her because
from their point of view, she’s only been away for a few seconds. Cara swears she was out for
longer and gives them her camera so they can see everything she saw. In return Taylor shows her his
own video, proving it’s only been a few seconds for them. The group finally stops being in denial
and accepts that time passes faster outside the cave, which means they’ve been stuck here for
weeks. The ropes haven’t been cut off, they wore off with time and friction, and Taylor’s theory
is right: the flickering light is the sun going through the day quickly. This is the Fountain of
Youth because they’re stuck here as youngsters forever. The group really wants to get out of
here so Taylor checks Furby’s body to get his rope, only to realize something: he hadn’t landed
on top of the radio, yet it’s still smashed. They decide to rewatch Furby’s video again and discover
that after the fall, he wasn’t dead yet, only unconscious. It was he who answered the radio when
they called, his voice sounded different because of the damage on his neck. Suddenly a mysterious
person appeared off-camera and killed Furby. Now the group is even more desperate to leave,
so Cara tries to climb out once more. While the group waits, Veeves checks Furby’s video again
and notices that the sky is changing its color. This means each flicker isn’t a day, it’s a whole
season, and what Cara saw outside was a destroyed Earth in the far future because they’ve been down
here for years. Suddenly something falls through the hole: it’s some sort of futuristic ladder.
Clara tries climbing it, but she has to go down when she notices a person in armor coming
down. Before they can try to communicate, someone attacks the person: it’s a caveman,
who made the noises they heard earlier and killed Furby. A fight ensues, but the futuristic guy puts
on a special collar on the caveman to subdue him before turning toward the group as he takes away
the stairs. Terrified, the group starts running away and doesn’t stop until they see some light.
They find a few lit torches and two bodies on the ground: it’s Hopper’s parents, who have been
down here for decades. To make matters worse, there’s a bunch of cavemen nearby feeding on
the dead cowboy, and they hear the group arrive. Taylor tells the girls to go while he fights the
cavemen to buy them time, but although he offers a fair fight, there are too many of them and they
knock him down. The girls hide behind a rock and see the cavemen run by, so Cara sneaks back and
is horrified to discover Taylor is dead. At that moment the futuristic guy appears and tests a pool
of water at the back of the cave. When he gets a positive result, he drags Taylor into the water,
pushing Cara away when she tries to take her friend back. At that moment a caveman shows up,
and the guy quickly knocks him down while Cara shockingly watches Taylor come back to life,
confirming this is the magic fountain from the stories. Soon the other cavemen also come and
by attacking the guy as a group, they manage to remove his helmet, which makes it hard for him to
breathe. The guy manages to put another collar on one of the cavemen, and for the third one,
Cara hands him back the tool he dropped so he can win the fight. Those futuristic tools put all the
cavemen to sleep. Afterward, Taylor tries to give him back his helmet so he can breathe,
but the guy shows them he’s out of time. At that moment Veeves and Jackie join them and since the
futuristic guy can’t speak their language, he plays some recordings on his computer glove so
they can understand what happened: when the group disappeared, their case was all over
the news and their families were pleading for help. Several thousand years passed since then
and Earth became uninhabitable, so humanity left in a giant spaceship called The Arc which is what
Care saw in the sky earlier. After the video is over, the guy dies. While discussing what to do,
Taylor finds something else in the cave: it’s Hopper’s bag. Determined to find him, Taylor tells
the girls to bring over Furby’s body while he goes looking for their professor, bringing the cowboy’s
gun just in case. He goes deeper into the cave and finds the trail of lights Hopper left behind,
which he follows until he finds some drawings on the wall representing the effects of the
cave. When he turns around, Taylor discovers a whole prehistoric family, but only the kid
is alive. He ignores them and keeps going until he finally finds Hopper, who is injured on the
floor. In front of him there’s the most shocking thing of all: there is a sphere combining several
layers of time and trapping people from different eras, including cavemen, the Conquistadores, and
Hopper’s sister. The professor explains that the whole cave system exists to protect the Fountain.
At that moment the girls start calling for Taylor so he tries to bring Hopper back with him, but
Hopper thinks he won’t survive the trip. He keeps the gun and tells Taylor to go without him. While
the prehistoric kid heals his parents with some water, Taylor reunites with the group and sees
Jackie healing her leg in the water. They throw Furby’s body in the fountain and while they wait,
they hear a shot: it’s Hopper, who failed the only bullet he had. The prehistoric family
comes after the group, so Veeves starts a fire to slow them down. They make it back to the room
with the hole and use the guy’s futuristic ladder to start climbing their way out. Cara reaches the
top first only to discover that the hole has been filled with water. When she touches it,
a mysterious creature grabs her up and pulls her through the water. Taylor tries to save her,
but at that moment a caveman presses the button on the ladder and makes it disappear, causing the
group to fall as Cara is taken out of the cave and the creepy hands put a special mask on her
face. The group thinks it’s over but in just a few seconds, strange cables come out of the water
and save the friends from falling. The cavemen try to jump to grab them, but at that moment,
a completely changed Cara shows up and begins pulling at the cables, dragging the group through
the water. After they are gone, more cables come down to search the caves. Moments later,
Furby wakes up and discovers he’s inside The Arc flying through space. Soon Hopper appears next
to him and the group comes to check on them, showing they’re dressed all futuristic too. On
another pool, Hopper’s parents also show up, and they’re welcomed by his sister who is fully healed
now. Now everyone can travel together to Mars with the rest of humanity to start a new civilization.