Meg Murry, Charles Wallace Murry, and Calvin O'Keefe  embark on a journey through space and time, from galaxy to galaxy, as they endeavor to rescue the Murrys' father and fight The Black Thing that has intruded into several worlds.

The novel offers a glimpse into the war between light and darkness, and good and evil, as the young characters mature into adolescents on their journey, and wrestles with questions of spirituality and purpose, as the characters are often thrown into conflicts of love, divinity, and goodness.