Invictus is the third novel in The Sebastian Cycle quadrilogy of the Blood of Martyrs series, which deals with the persecution of Christians during the reign of Emperor Diocletian. The main characters are historical figures known in hagiographic literature as Saints Sebastian and Susanna.

As Emperors Diocletian and Maximian are entering Rome, Sebastian and Susanna mourn the recent martyrdoms of their friends.

With this weighing on his conscience, Sebastian must welcome the Emperors with a cheerful face, even as local authorities are zeroing in. Ironic given that Diocletian is promoting him.

Meanwhile, Emperor Maximian is making lecherous eyes at Susanna. But he is not the only one. Torquatus, the betrayer, is lurking the palace, looking for blood.