From the outset, the double-booking is not an administrative glitch: Will and Cin engineered the entire encounter. As the weekend progresses, Will covertly steals Diego’s engagement ring from his luggage and, during a tense dinner, uses that exact ring to ostentatiously 'propose' to Cin, mocking Diego’s private intentions while testing his courage to speak up.
The major revelation surfaces in the third act: Will and Cin are serial killers and incestuous siblings/lovers. Driven by pathological narcissism and jealousy, they hunt couples whom they perceive as either trying to project marital perfection or struggling with domestic friction. They view dismantling these relationships from the inside as their personal 'art piece,' orchestrating infidelity and paranoia before executing the victims.
When Diego and Sage attempt to leave, the house becomes an active slaughterhouse. The confrontation moves onto the water in the film's climactic boat sequence. When Will overpowers Diego, Sage intervenes and kills Will in cold blood to save her partner. Diego violently fights off Cin, severing her ring-bearing fingers before starting the boat’s engine and running Cin over, chewing her to pieces with the propeller.
In the final scene, Sage and Diego drift on the blood-splattered boat in complete, stunned silence. Diego retrieves his stolen engagement ring from Cin's severed finger, wiped clean of gore, and silently offers it to Sage. Sage accepts the ring and slips it on, closing the film not on a triumphant note of romantic rescue, but on a shell-shocked tableau of two survivors forever altered by the violence.