The plot is triggered by Quevedo's request to Alatriste for help in rescuing a girl who has been forced to enter a convent. Iñigo climbs into the convent and opens a door to Quevedo, Alatriste, and the girl's family members, but they are caught in a trap and Alatriste and Quevedo barely escape with their lives. Meanwhile Iñigo, who has disobeyed Alatriste's instruction to return home, is captured by Alatriste's old enemy, the sinister Italian swordsman Gualterio Malatesta, who delivers him to the Inquisition.
Quite a lot of the book is taken up with Iñigo's experiences at the hands of the Inquisition. These are suitably horrific, although luckily he is not tortured on the rack because he has not quite attained the age of fourteen. (The Inqusition's rules did not allow torture of children below this age.)
The question of 'purity of blood' which gives the book its title, relates to people whose forebears had been 'conversos' (Jewish converts to Christianity) and who were suspected of backsliding. If convicted of this crime they were liable to execution by burning. The central event in the novel is an 'Auto de Fe', which is staged in Madrid with the King and Queen in attendance. Iñigo is one of the accused, alleged to have taken part in Jewish rituals, and is now awaiting sentence.
As usual, there is plenty of drama and sword-play, which on one occasion takes on a near-farcical character, when Alatriste breaks into the house of Luis de Alquézar, Angélica's powerful uncle and guardian, intending to terrify him into getting Iñigo released. But Angélica comes on the scene and attacks Alatriste savagely, scratching him and biting his arm.
There is a lot of local colour, with depictions of seventeenth-century Madrid low life. Sometimes the details of this may be obscure to readers who lack some background information. For example, Chapter III describes Iñigo's encounter with Angélica in the 'Acero' district of Madrid. This was a place where water containing iron was drunk for medicinal purposes ('acero' = 'steel'), but in the seventeenth century 'tomar el acero' ('to take the steel'—compare 'to take the waters') could also refer to the making of romantic assignations.
As in the previous book, Iñigo, who narrates these events, includes a good few verse quotations, mostly from Quevedo, in his story, along with political reflections on the sorry state of decadent Spain. Fortunately these don't hold up the action too much. The principal characters, Alatriste and Iñigo, continue to develop in a convincing manner.
The final episode in the book has Alatriste encountering Malatesta, with whom he had fought a few days previously, now lying in bed seriously injured. Alatriste wants to kill him but can't bring himself to do so while the man is defenceless. And he is forced to recognise that he and Malatesta have more in common than he likes to admit.