Something is very wrong with Ax. He's sick. And the Animorphs don't know what to do about it. They can't take him to a hospital. They don't know how to contact the Andalite home world. And Ax is dying. Cassie and the other Animorphs come up with a plan. But it involves finding a new way into the Yeerk pool. There's just one problem: Jake, Rachel, Tobias, and Marco come down with Ax's sickness. And now, for the first time, Cassie is on her own. . . .