Fifteen-year-old Vicky Austin and her family are spending the summer on Seven Bay Island with her maternal grandfather, who is dying of leukemia. At the beginning of the story, Vicky attends a funeral for Commander Rodney, a family friend. Also present are the commander's wife, his sons Leo and Jacky who own a launch boat business, and Adam Eddington, an intern at the Island's research base and friend of Vicky's brother, John.
After the funeral Vicky encounters Zachary Gray, her boyfriend from the previous summer whom her family does not particularly like. She soon learns that Zachary indirectly caused Commander Rodney's death; the commander had his heart attack while saving Zachary from a suicide attempt. This revelation and others set Vicky on a train of thought that continues throughout the book; the mysterious and (to Vicky) frightening topic of death. Death and the threat of it seem to loom everywhere, from news reports to the death of a baby dolphin, from the recent demise of Zach's mother in an automobile accident to Grandfather Eaton's slow deterioration.
During the course of the story, Vicky finds herself in a tangle of three romances; one with the solid, unexciting Leo, one with dark and dangerous Zachary, and one with the gentle but emotionally damaged Adam, whom she is helping with a project on dolphin and human communication (ESP) with three dolphins: Basil, Norberta, and Njord. Vicky discovers a remarkable rapport with the dolphins, an unspoken communication that teeters on telepathy. Her ability extends to communicating with Adam as well, but he pulls away, unwilling to allow that level of intimacy after a devastating betrayal the previous summer.
Meanwhile, Vicky must help out at home, facing her grandfather's increasing confusion as he identifies Vicky with his dead wife. He has also been hemorrhaging, and Vicky often goes with Leo to pick up blood. There at the hospital, she meets a girl named Binnie who is sick with a type of leukemia and has seizures. Binnie's father is radically religious and is constantly disposing of the medication that control the seizures.
One night, her grandfather starts to hemorrhage and is sent to the hospital. Vicky is on a date with Zachary, and does not know about her grandfather's medical crisis until they come to the dock and see that Leo is not there to pick them up. Zachary rushes Vicky to the hospital, and eventually abandons her there. As she waits in the emergency room, she is spotted by Binnie's mother, who leaves her unconscious daughter with Vicky while she goes to find a nurse. Binnie has a convulsion and dies in Vicky's arms. This latest trauma sends Vicky into a wave of darkness, an almost catatonic state in which she is only vaguely aware of reality. Vicky's parents and Leo, who are already upset because Vicky's grandfather has been bleeding internally, try unsuccessfully to comfort and communicate with her. Then she feels hands on hers - Adam's. He tells her that she "called" him (meaning with ESP) and he came.
The next day, Vicky is still in a wave of darkness. Her grandfather tells her that it is hard to keep focused on the good and positive in life but she must bear the light or she will be consumed by darkness. He also removes the emotional burden he placed on her earlier, when he asked her to tell him when it was time to die. Vicky is unable to listen, too caught up in her own misery. Finally Adam takes her into the ocean, where Vicky's dolphin friends break through her mental darkness, until she is able to play with them and face the light again.