The Message, published in October 1996 and written by K. A. Applegate, is the fourth book in the Animorphs series. It is narrated by Cassie.

Cassie and Tobias are having strange dreams about a presence in the ocean. Jake sees a news item on television about debris with what looks like Andalite lettering on it that has washed up on the beach, and when he shows it to the others, Cassie and Tobias have such strong visions that they momentarily pass out. The Animorphs decide to investigate, and acquire dolphin morphs to do so.

While out in the ocean, they find a humpback whale under attack by a group of sharks. The Animorphs fight the sharks and drive them off, and Marco is nearly killed in the process. Marco is able to morph back to his human form, and the whale, grateful, saves him from drowning. The whale speaks to Cassie through song, telling her about a strange place of grass and trees under the ocean. Cassie has a feeling that this place is of Andalite origin, and the Animorphs wonder if Cassie's and Tobias's visions are a sort of distress call from an Andalite trapped in the ocean. They decide that the distress call is connected to the morphing ability, and Cassie and Tobias feel it the strongest because Cassie is the most in control of her morphing ability and Tobias is trapped in a morph. They also figure out that Visser Three could be receiving the message and that the Yeerks are probably looking for the lost Andalite as well.

The Andalite's location is too far from the shore for the Animorphs to reach under the two-hour morphing limit, so they (except Tobias) morph into seagulls and stow away on a large container ship (called the Newmar, from Monrovia, headed towards Singapore) that is heading in the right direction. They abandon ship and morph to dolphin when they are in range and discover the dome of an Andalite Dome Ship deep beneath the ocean's surface. They enter through an airlock and meet Aximili-Esgarrouth-Isthill. Marco quickly gives him the nickname Ax, and the Animorphs tell him that he is the only survivor of the Andalite-Yeerk battle in Earth orbit and that they met and received the morphing power from Prince Elfangor. Ax reveals that Elfangor was, in fact, his older brother.

The meeting is brief however, as the Yeerks discover the sunken Dome ship and begin to drop depth charges. Ax had previously acquired a tiger shark, and he and the Animorphs escape. They are pursued by Visser Three in Mardrut morph; as they tire, Visser Three gains on them. The Animorphs decide to make a last stand, but are saved when Visser Three is attacked and chased off by a pod of sperm whales. The whales give the tired Animorphs a ride back to the shore.

Ax pledges to fight with the Animorphs and adopts Jake as his prince. He acquires Jake, Rachel, Cassie, and Marco and mixes the DNA from each to create his own human morph. The Animorphs decide to hide him in the woods near Cassie's farm.

Ax is introduced.

Jake -Trout, bottlenose dolphin, seagull.

Rachel -Trout, bottlenose dolphin, seagull.

Cassie -Squirrel , trout, bottlenose dolphin, seagull.

Marco -Trout, bottlenose dolphin, seagull.

Ax -Tiger shark, Human (blending Cassie, Marco, Rachel, and Jake's DNA into one strand).

Visser Three -- Mardrut