"Horizon Line" follows former lovers Sara and Jackson (Allison Williams and Alexander Dreymon) who find themselves reunited ahead of their friend's tropical island wedding. As the only passengers on a single-engine Cessna plane making its way over the azure waters of the Indian Ocean, a spark soon rekindles. However, when their pilot suddenly suffers a heart attack, the couple must fight for their lives to stay airborne and find their way back to land.