Nancy, Bess, and George are off to Key Largo, Florida for a week of sun, swimming and exploring the fabulous coral reef that stretches along the Florida keys. They've been invited by Delia Duke, a friend of theirs who is a marine biologist at the John Pennekamp Coral Reef State Park. The park is a scuba diver's delight, with dazzling corals and tropical fish, and Delia promises to show the girls the best snorkling and diving spots. But as they head through the mangroves towards her boat, someone takes off in a small motor boat, leaving behind a bag filled with rare, endangered butterfly orchids. This is just the beginning of a shocking eco-mystery that'd be too complex for anyonebut Nancy Drew. . .