When protagonist Rupert Venables manages to solve the password protection levels of the late Emperor Timos IX's secret computer files pinpointing how to find the heirs to the imperial throne of the Koryfonic Empires, he discovers a variety of visual imagery including pictures of various centaurs and humans, and the name "Knarros" which seems to be associated with a group of heirs. The problem: nobody knows who or where this "Knarros" might be.
Luckily, the High Lady Alexandra is on the job. The only surviving one of the late emperor's Imperial Wives, Lady Alexandra is smart, observant, and socially skilled. She is sent by the acting regent, General Dakros, to his home world of Thalangia, where she talks with his people and makes connections between bits of information she hears.
One such bit of information is that there is a reclusive group of young people living in a closed compound not too far from the house where she's staying. Another is that the compound is run by followers of the late emperor's bush goddess Aglaia-Ualaia. And the third bit is that the place is run by a cranky, antisocial brute of a centaur named Knarros.
And it turns out that indeed, they have found the missing Knarros. He is a hugely tall, grim centaur whose skin, hair and mane are all uniformly dark slate gray, which increases his resemblance to a statue.
Rupert himself, responding to a message directing him to go to the compound as the official Magid-in-charge of that group of worlds, embarks in his car with Stan on a world transit to Thalangia, meets up with Dakros, and makes his way through many layers of magical protection up to the gate, which he eventually levitates past, into the compound. There he meets a number of young humans, young Centaur Kris, and eventually Knarros himself, who constrains Rupert to perform the entire complicated Magid verification ritual.
Shortly after he has finished, a series of extremely unpleasant and shocking events ensue, which change everyone's understanding of what has been happening at that location, and which set in train the magical quest that forms the climactic focus of Deep Secret.