Two American couples—Amy and Jeff, Stacy and Eric—are visiting Mexico for three weeks in August on vacation. They decide to go snorkeling off the coast of Cozumel. When the guide is unable to find the wreckage from a ship, they spend the afternoon swimming in the sea. They run into a German man named Mathias who is out diving. He lends them his tank so they can see the wreckage, then joins them when they return to the shore.

Two days later, the couples meet three men they call “the Greeks.” None of them speak English, and they adopt the Spanish names Pablo, Juan, and Don Quixote. They repeatedly show up throughout their vacation.

One morning, Mathias approaches Jeff and Amy about his missing brother, Henrich. Henrich met an archaeologist who was there to excavate a dig, and she invited him to join her. Mathias and Henrich fought about it, but Henrich insisted that he was in love and needed to follow her. He left Mathias a note with a map to the site, which is an 11-mile drive from Cobá followed by a two-mile hike from the beach.