Lucy's big break, a London play, had folded after two months, and so she is vacationing with her married sister in Corfu -- a Greek island, across the shore from Albania. (Still Communist Albania, this being a period piece.)
She goes down to swim on the shore, and a dolphin comes to swim with her -- when someone tries to shoot it. Lucy jumps into the water to frighten them with the prospect of shooting her, and then storms up the cliffside to the house which Sir Julian Gale, a famous actor, is renting. She finds a man in the garden and bitterly accuses him; though cold and unwelcoming, he manages to convince her that it wasn't him. She later hears that Sir Julian had a breakdown, and that the house there is filled with weapons.
Meanwhile, a Greek boy drowns while helping an Englishman with his photography at sea; the body is swept away. A fact which bears down hard because his mother and sister work for her sister, the photographer is their other neighbor, and Sir Julian is the boy's godfather, a big deal in those parts. And through blundering into the rose garden, she gets to hear Sir Julian's theory that Corfu is the original location of The Tempest.
Another drowning makes the plot thicken, into a story involving caves, smuggling, Greek antiquities, a diamond, the dolphin again, the feast of the local saint St. Spiridon, and many more plot twists.