Winter's surrogate mother, the very elderly dolphin Panama, has passed away, leaving Winter without the only poolmate she has ever known. However, the loss of Panama may have even greater repercussions for Winter, who, according to USDA regulations, cannot be housed alone, as dolphins' social behavior requires them to be paired with other dolphins. Time is running out to find a companion for her before the team at Clearwater loses their beloved Winter...

When Ruka was younger, she saw a ghost in the water at the aquarium where her dad worked. Now she feels drawn toward the aquarium and the two mysterious boys she meets there, Umi and Sora, who were raised by dugongs and hear the same strange calls from the sea as she does.