Late in the third century, under the rule of the barbarian Diocletian, began the last and one of the most severe persecutions of Christians in early church history. During this time Sts. Sebastian, Agnes, and Tarcisius, among many others, won the martyr's crown. This book gives an account of their witness.

Fabiola is a Roman lady, a "noble pagan" of intelligence, virtue and spirit, who like many Romans of the time has heard nothing but false, vile reports of Christian beliefs and practices. At the same time she can't help contrasting the behavior of her young cousin, Agnes, her friend Sebastian, and her slave Syra with the corrupt brutality so rampant in pagan society. Without knowing that they are Christians, she senses that they are in light while she is still in darkness.

While Fabiola and her journey towards conversion is the focal point of the book, there are as many turns and side passages in the plot as there are in the catacombs. There is a subplot concerning an immature Christian who falls into apostasy and betrays his friends - but repents and is restored to grace. Another subplot concerns a martyr's son who longs to follow in his father's footsteps. A chapter is devoted to explaining the purpose of the catacombs and how the inscriptions came to be written within them. There is a lot to be learned from this book about an important time in Christian history. The descriptions of the martyrs' deaths are realistic, and because of this, and the length and ornate style of the book, it might be best read by teenagers and adults.