The Message, the fourth novel in K. A. Applegate's "Animorphs series," features five young human friends who can take on the shape of animals. Those who have this ability are called Animorphs.
The novel opens with protagonist Cassie morphing into a squirrel one night to investigate the fox that has been taking the rescued birds she and her father, a lead vet, work so hard to care for in their family barn they have converted into the Wildlife Rehabilitation Clinic. Though Cassie as a squirrel tries to chase off the fox, she winds up needing to be rescued by her best friend Tobias who, appearing in the shape of a hawk, swoops down and chases off the fox.
In the barn, Cassie and Tobias talk about how they have both heard in their dreams a voice crying out for help, coming from "under the sea" (p. 55). When Cassie and Tobias discuss the dream with their other three anamorphic friends, one friend points out a news story about a guy on the beach finding a piece of metal containing unusual writing. The friends agree that the writing is the same as the writing found on an Andalite spaceship and that it is an Andalite calling out for help in the ocean. Yet, they also realize that both the Andalite and their group are in danger of being pursued by their enemies the Yeerks if they try to rescue the Andalite. However, the group agrees that the rescue mission must be pursued and promptly morph into dolphins.
Once at the bottom of the ocean, they find the Andalite, called Ax, in a park covered by a "plastic dome" (p. 105). Though they must battle the Yeerks, especially Visser Three, the Yeerks's evil leader, they are able to rescue Ax but not without the help of a whale they had rescued from sharks earlier.