This book, "Doctor Who: The Dimension Riders" by Daniel Blythe, tells a complex science fiction story involving the Doctor and his companions, Ace and Bernice Summerfield. The narrative centers on a dangerous entity called the Garvond, which manipulates time and preys on the fears of its victims. The plot unfolds across various locations and time periods, including a space station (Q4), Oxford University, and within the Time Vortex. Characters like Captain Terrin, Lieutenant-Commander Quallem, Professor Rafferty, and Tom Cheynor become entangled in the Garvond's schemes, which are orchestrated by a renegade Time Lord known as Epsilon Delta, posing as the President of St. Matthew's College. The Doctor and his allies must work to understand and ultimately thwart the Garvond and Epsilon Delta's plan to disrupt time and gain immense power.