A young, mild-mannered American couple, Cliff and Cydney, are celebrating their honeymoon by hiking to a secluded beach in beautiful Hawaii. Cliff, an aspiring screenplay writer and film school graduate, is eager to document their journey, while Cydney captures the moments on camera. During their adventure on the Kalalau trail, which spans 11.5 miles each way and takes two days to complete, Cydney notices an unexplained scar just below Cliff's chin, prompting curiosity.
As they begin their hike, the couple encounters two hitchhikers, Kale and Cleo, but Cliff declines to offer them a ride. Despite Cleo's friendliness, showing Cydney her wedding pictures, Cliff senses trouble and insists they move on. Their hike takes a turn when they meet Nick, a solo hiker who dramatically saves Cydney’s life when she slips on a narrow ledge.
Things become unsettling when the trio overhears a conversation among a group of frightened female hikers discussing a recent double murder of another couple on the island. Cydney and Cliff contemplate whether they should abandon their trek, especially since authorities are on the lookout for a man and a woman linked to the gruesome crimes. However, they decide to continue with Nick, who claims to be a veteran of the Iraq War. He shares chilling tales from his experience, including an injury that left him with a titanium plate in his skull, suggesting that brain injuries don't hurt because of the lack of nerve endings.
Suspicion grows as Cliff starts believing that Kale and Cleo might not be the only dangers lurking. Nick's increasingly erratic and unsettling behavior raises alarms about his possible connection to the murders. In a twist of fate, Cliff discovers that the permit for their hike is missing, leading to further paranoia as they suspect they are being followed.
As the plot unfolds, Cliff's doubts mount when he uses his phone to search for images of the murder suspects, discovering eerie similarities between them and Nick and Gina. When tensions peak, Cliff encounters a guide who hands him the permits they thought they lost, and they continue on their trek. A horrifying discovery awaits them—Kale and Cleo are arrested, but sinister truths begin to surface.
At the beach, Cliff and Nick embark on a kayak adventure to explore a marine cave, unaware of the perilous situation that awaits them. Meanwhile, as Gina, who accompanies the group, views footage on Cliff and Cydney's video camera, she realizes that they are not who they claim to be.
Flashbacks reveal the chilling reality: Cliff, who is actually Rocky, and Cydney are a drug-addicted couple masquerading as innocent honeymooners while they commit heinous acts of murder to steal identities, allowing them to lead a hundred different lives and attain a twisted sense of immortality.
In a climactic scene, Rocky confronts Nick in the cave, resulting in a frantic and violent struggle. Cydney gets involved, leading to a desperate attempt for survival. A frantic call to the police leads to a tense standoff where unexpected alliances form. The police finally arrive with a helicopter as Rocky is confronted and ultimately shot dead by law enforcement.
In the aftermath of the chaos, Nick and Gina are airlifted to safety, where Nick proposes to her in the helicopter, humorously admitting he purchased the ring on eBay. They share a moment of relief and love—agreeing that their honeymoon experience has certainly been anything but ordinary with the declaration of "No honeymoon."
