How much more could be learned about using and conserving ocean resources if humans could communicate with intelligent marine creatures? This question is sure to intrigue readers who follow the adventures of Mo and Duke, a lonely girl and a dolphin who develop a deep friendship when they learn how to use whistles and mental linkages for communication.

Tons of human waste are dumped into the oceans each year. Readers of this story become aware of the high environmental cost of this practice when Duke, a curious young dolphin, is poisoned by contact with poisonous blue slime in the hold of a sunken ship. Paralyzed and unable to swim, the dolphin's pod brings him to the Duke University Marine Lab dock, where they hope the people of the land can help. A dolphin rescue team there saves his life.

Because his skin is a bright blue from contact with the blue slime, the paralyzed dolphin comes to be known as Duke's Blue Dolphin. Mo, a lonely young girl who lost her ability to talk in a freak plane accident, has learned sign language and is home-schooled on the lab's island by her caring parents. She has no friends other than Fetch, a big black retriever. She is strongly attracted to the sick blue dolphin whose hammock in a dock-side enclosure enables him to breathe. Despite her parent's objections, Mo refuses to leave the dolphin's side and stays by him night and day. The rescue team dwindles in numbers when the dolphin doesn't try to eat or swim. The leaders of the rescue team hesitantly accept Mo's help to keep Duke alive. Although she is much younger than the others, Mo becomes an important part of the team, keeping his skin moist and feeding him daily.

Mo's heart is touched when she senses that the dolphin, cut off from his pod, shares her sad and lonely feelings. Although she can't talk, she can whistle. Mo knows that dolphins are social creatures and whistle, ping, and click to each other. To ease his loneliness, Mo whistles to Duke as she cares for him. Duke slowly regains his health and notices the young girl and her whistles. One day Duke whistles back, beginning their remarkable whistle-talk communication and growing friendship.

On one dark night, Mo goes out into a fierce storm to check on the safety of her dolphin friend. As she does this, she overcomes her extreme fear of the loud sounds that always make her mind replay the plane accident that robbed her of speech. Duke sees Mo and breaks out of his hammock when he thinks she is in danger, swimming for the first time since being poisoned. It is, however, the rescue team leader who is in danger. While seeking to keep Duke's enclosure intact during the storm surge, he contacts the poisonous blue slime on a loose timber and loses consciousness. Duke and Mo share a strong mental linkage and save the life of the rescue team leader.

A time of great joy and happiness for Mo and Duke follows the storm. Duke swims, leaps, and chases toys thrown to him. He gives Mo and Fetch rides on his back and plays games with them. Mo and Duke further develop their whistle talk and occasionally share mental linkages. Mo shows Duke computer images that tell him about her life on land. Duke sends her mental images of his life in the sea. Never have humans and dolphins shared this way. Mo and Duke keep their ability to communicate a secret.

Mo grieves when it's time to set Duke free. Will Duke come back to visit her after he is released? Mo hopes so. She only knows that she would not forgive herself if Duke were kept captive for study in a research facility because she revealed their communication ability.

Mo bravely faces the loss of companionship of the dolphin she has come to love. When Duke locates his home pod, Mo gets a final ride with her friend and lets him go. Duke becomes a free dolphin once again.