Hopper (Andrew Wilson), an archaeology professor, is exploring a remote cave system on the trail of missing hippies from the 1970s. Hopper finds the hippies van. After discovering what appears to be a cowboy paused in place in a tunnel, he returns to town and dismisses his graduate students, Taylor (Reiley McClendon) and Jackie (Brianne Howey), stating that their research is done.Hopper enters the cave system from the opening where he found the cowboy. Hopper finds an invisible barrier inside which the air is measurably damp from the outside.Several days later, as Hopper has not returned, Taylor and Jackie go looking for him, with their friend Cara (Cassidy Gifford), who brings her sister, Veeves (Olivia Draguicevich), and her friend Furby (Max Wright). They find the van, which Hopper discovered. From the photos left in the van, Taylor and Jackie find out that the missing hippies were Hopper's parents.Following Hopper's trail to the campsite by a cave, they find an entrance with climbing ropes leading inside. They decide to follow it, but Furby chooses to stay behind as backup for the base camp. They lower themselves into the cave, hearing strange noises & encountering the same invisible barrier as Hopper.As Hopper enters the cave, the cowboy walks away normally, and when Hopper looks out he sees the shadows of sunrise and sunset pass by several times in a matter of seconds.Hopper comes out of the cave, only to find his jeep overgrown by vegetation, and his dog nowhere to be seen. He finds Taylor and Jackie's car nearby and it seems that it has been there for several years. Hopper goes back into the cave to find Taylor and Jackie.When Furby does not answer their radio calls, Jackie tries to climb back out, but the rope frays, and she falls, injuring herself and Taylor. They think that Fury cut the ropes. Calling for help, they receive a transmission from inside the cave and follow it. The message was "help me" in a very faint voice, who claims to be Furby.At a different junction, they discover another opening to the cave and Furby, dead with a broken neck. Watching his video recordings, they notice several days elapsed from his perspective, while only about an hour had lapsed in the cave. He began researching the hippies' belongings, discovering that Hopper is searching for his missing sister (when the sister got sick is when Hopper's parents went about looking for the Fountain of Youth). His parents were following legends of the Fountain of Youth and speculated that this cave system was the root of the story. Furby found another cave opening. After several days, he went into the cave to retrieve keys for a vehicle but fell when his rope severed.Taylor deduces the cave is inside a time distortion where events move more slowly. With few options, Cara free-climbs out of the cave to get a GPS signal. Outside, the terrain has become barren, and there is no signal. There are massive dust-storms and Cara notices something massive orbiting the EarthReturning to the cave, comparing video recordings, Cara has experienced 30 minutes on the surface while she had been gone only a few seconds from their viewpoint, confirming Taylor's theory. Reviewing Furby's footage again, they learn he survived the fall but was murdered by a caveman. Cara and Taylor deduce the time difference is much more drastic than they suspected and entire years are passing within seconds, meaning the few hours they have been in the cave is enough to span several hundred or thousand years outside.Hearing more howling, Cara prepares to climb out again for help but is interrupted by a futuristic humanoid spaceman descending via a retractable ladder. A caveman suddenly attacks, but is subdued by the caveman non-lethally. They flee just to find a whole tribe of cavemen inspecting the dead bodies of the cowboy and Hopper's parents. Discovered, Taylor fights the cavemen and is killed.The spaceman returns, protects the others from the cavemen, and then places Taylor into a healing pool of water. More cavemen attack, and the spaceman is fatally wounded, but uses his dying breaths to show them several media clips about their disappearance and indicating that humans have become space-faring.Taylor finds Hopper injured in front of another time dilation, containing his long-missing sister and a legion of conquistadors in battle with the cavemen over control of a waterfall, the source of the time distortion, at the center of the cave system. Hopper explains the field is strongest here, making rescue of his sister impossible; he tells Taylor to go and save the others, as he is also dying. Preparing to leave the cave with the spaceman's ladder, the students are attacked by cavemen before they can get through; Cara is pulled through the portal by future beings before she can help her friends. From their perspective, she reappears through the portal, differently dressed and with rope-like mechanisms which pull her friends through.A short time later, Furby awakens, along with Hopper and Hopper's family, all having been retrieved and resurrected with the healing waters. The others arrive, happily explaining they are in a space station and have a lot to talk about.