The Alien is the eighth book in the Animorphs series, written by K. A. Applegate. It is the first book narrated by Ax. Published July 1997 .

After destroying both the Kandrona and the Veleek, the Animorphs assumed that they would see people freeing themselves of the Yeerks. They are disappointed until the day they take Ax to the cinema. A man's Yeerk is seen dying publicly. However, a Controller-policeman kills the free man.

The Animorphs take Ax to the school as Philip, Jake's cousin, and a Yeerk who controlled one of Jake's teachers is seen dying as well. Chapman appears, orders the students to leave, and kills the non-Controller teacher.

Jake and the other Animorphs become very angry with Ax because they feel betrayed. Innocent people are dying as a result of their actions. Ax retorts that they would not have destroyed the Kandrona had they known the consequences, to which Jake replies that Ax still has a lot to learn about humans.

The next day, he meets with Marco to go to a bookshop in hopes that Ax would trust them if they trusted him. However, Marco forgets the money they collected for him to buy a book at home, so he and Ax go to Marco's house to pick it up. While Ax waits for Marco in the living room, he plays what he thinks is a game on Marco's father's computer called "Fix the mistakes." He ends up messing up the computer.

It turns out that he had developed a new system that was very advanced. Before destroying it, he used it to communicate with his home world. There, an Andalite made him assume all the responsibility for Elfangor's action and is consequently forgiven. When he was about to speak with his parents, he is interrupted by a Controller whose loved one had died when Visser Three chose to sacrifice her after the Kandrona's destruction. To avenge her, he tells Ax where and when Visser Three feeds his Andalite body.

Ax decides to go alone and not tell the others about the information he received. He poisons Visser Three by morphing into a rattlesnake and biting him. As Ax is about to die, the Animorphs arrive to save him. With his host body having been poisoned, the Yeerk Visser Three leaves it. However, Ax is unable to kill a fellow Andalite, so Visser Three's host Alloran-Semitur-Corrass asks him to tell his family that he is still alive and that he has not lost hope.

Ax returns to the observatory, calls his home planet, and delivers Alloran's message. He announces Earth is his new home, and that he will tell the Animorphs everything.

Ax tells the Animorphs that Seerow was the first Andalite to go to the Yeerk home planet, and that he felt sorry for the Yeerks and gave them the technology they later used to conquer the world. Contrary to what Ax had expected, the others didn't blame the Andalites for their problem. They recognized the good action and told him to keep trying, but to be more careful the next time.