After their boat sinks during a storm, Jennifer Remming washes ashore a small, tropical island with her friend Brad, who dies from his wounds shortly after. While exploring the island, Jenn discovers the belongings and graves of a family that once lived there. She later buries Brad’s corpse in the sand, but discovers it savagely eaten and mutilated the following morning. The next day, while retrieving her newly surfaced luggage, Jenn finds an ominous hole in the ocean’s floor. That night, Jenn fails to flag down an airplane using a flare gun and encounters a giant, two-legged sea monster. For the next four nights, Jenn manages to hide from the monster while also attempting to catch and kill it. As Jenn prepares for the fourth night, the corpse of another mutual friend, Zack, washes ashore mutilated and bisected. Jenn later uses his body as bait, seeing the monster clearly for the first time. For her fifth attempt, Jenn sleeps in a makeshift hammock in the trees to watch the monster arrive. When the hammock falls, Jenn stabs the creature with a sharpened stick and narrowly escapes. The next day, Jenn is reunited with her boyfriend Lucas Griffin and friend Mia Reed after the two wash ashore in a lifeboat. Jenn warns Lucas and Mia about the creature, but they dismiss her claims. Jenn later discovers Lucas’ pocketknife bloodied. When Jenn makes a desperate attempt to flee without the two, Mia knocks her out with a boat paddle. Later, Jenn regains consciousness, finds herself tied up, and tries to defend her actions. Mia reveals Jenn’s past of fabricated lies, including one about being mugged before the group’s trip. When Mia leaves, Lucas hints to participating in Zack’s demise. Before Jenn can ask what happened to their friend, Mia is attacked by the monster. Lucas leaves Jenn tied up to fight it himself, but fails to save Mia, who is dragged into the water. The next morning, Lucas and Jenn escape with the lifeboat, the latter discovering the inside covered in blood, presumably belonging to Zack. As the two head west, the sea monster attacks the lifeboat, eventually dragging Jenn away. When Jenn injures the monster, it releases her and drags Lucas to his death. Now alone, Jenn uses her journal to help any individual who may encounter the monster after washing up on the island. That night, Jenn sets some of the island on fire while battling the monster with a series of sharpened branches and bones from the graves of a family the monster killed. Both severely injured, the monster chases Jenn to the shoreline, but collapses from its wounds. Jenn decapitates the corpse and limps away to the raft, carrying the decapitated head as proof of what happened on the island, before presumably sailing west for rescue as the camera pans out, revealing that the island's foliage is now partially ablaze.
On February 2, 2017Blumhouse Productions announced that Kiersey Clemons was starring in the film with J. D. Dillard co-writing, producing and directing. On May 9, 2017 it was announced that Hanna Mangan-Lawrence, Emory Cohen, and Benedict Samuel would co-star.
