Canon Tallis, newly arrived at the Cathedral of Saint John the Divine after the events of The Arm of the Starfish, meets the Austin children and their friends just as they encounter an anachronistic Genie in a junk shop.

Tallis advises and helps to protect the children as they are drawn into a mystery involving the Genie, a street gang called the Alphabats, and the local bishop's strange behavior.

At the time of the book's publication, Kirkus Reviews said, "The Young Unicorns is a kind of aggiornamento of the Austin series and The Arm of the Starfish--via cross-references and a congeries of characters--and Wrinkle...the latter because seven-year-old Rob Austin speaks with the same precocious wisdom as Charles Wallace and, in the clinch, withstands evil with the same indomitable innocence: the roles are interchangeable... Miss L'Engle envelops these melodramatics in church music and theological speculations; she also writes with an insinuating slickness: the insupportable is readable.

"[4] Writing for Analog Science Fiction/Science Fact, P. Schuyler Miller said, "There's a distinct flavor of C. S. Lewis and Charles Williams to the religious side of the story.

The young people are real, the plot is good, the neighborhood comes to life—in short, here is a book by an extremely capable writer which is almost science fiction.

"[5] In a 2012 essay for Tor.com, American author and critic Mari Ness commented, "L'Engle's plotting is tighter here than usual... And if some of the themes of this book, her saddened observation of a rising tide of violence and hatred in the 20th century, her focus on the ability to choose between love and hate—have made regular appearances in previous L'Engle books, and will appear again, here they are woven together in a relatively engaging plot.

Vicky is the first person protagonist and narrator of the first two novels in the series, Meet the Austins and The Moon by Night, and of the book that follows The Young Unicorns, A Ring of Endless Light.