Anthony Whitelands, a young, rather prim and bookish English art historian and expert on Velasquez, is approached by an art dealer of dodgy repute to value some paintings belonging to one of his clients in Madrid.

He agrees to the venture in the hope that he might find some genuine treasures, and to put distance between himself and his fiancee Catherine – a relationship he has grown tired of.

It is the spring of 1936 and Spain is in political turmoil, with either a leftist revolution or a right-wing coup looming, but Anthony sees the country as noble and inspiring. As he puts it in a letter home: "In our country we always go around with our heads down, staring at the ground, feeling crushed; here, where the land offers nothing, people hold their heads high, and gaze at the horizon. It is a land of violence, passion, of grandiose individual gestures. Not like us, constrained by our petty morality and trivial social conventions."

The art collection belongs to the wealthy Duke of Igualada, but the paintings Anthony values turn out to be worthless. The duke's beautiful, bewitching daughter Paquita, however, shows him the real treasure which is hidden in the basement: a hitherto unknown Velasquez, and what's more it appears to be the artist's own personal – and more revealing – version of the Rokeby Venus. Anthony believes it to be genuine, which not only means that it is worth a fortune, but it will also guarantee his renown as an art historian for its discovery.

At first Anthony is led to believe the money raised by its sale will finance the family's relocation to the safety of England, then he learns that it may be to buy arms for the quasi-fascist group the Falange, led by Jose Antonio Primo de Rivera, a handsome and charismatic lawyer and a close friend of the duke's family. He is also Paquita's suitor, and Whitelands is drawn into vying for her favours with him.

The Spanish civil guard put Anthony under surveillance, as do Soviet spies, the Falange and the British Embassy. Anthony becomes the lens through which opposing factions can keep an eye on each other, and he is caught in a web of spying, rumour and deception. Mendoza skilfully weaves the changing allegiances and political complexities of the time throughout the narrative to create an atmosphere of distrust and paranoia, with the mounting threat of civil war dominating everyone's motives and actions. This atmosphere of foreboding in a country on the brink of civil war is superbly realised, though at times the narrative is burdened with overlong accounts of one political figure or another.

Nevertheless, this is an engaging comic novel in which our rather ridiculously naive innocent abroad, manipulated by others, finds himself in one predicament after another until his life comes under threat. For all his protestations that as an Englishman he has no opinion about the future of Spain and wishes to take no side, circumstances won't let him sit on the fence. Though the historic elements of the story are certainly authentic, some turns of the plot seem like unnecessary and unbelievable complications, as when Anthony becomes involved with La Tonina, a child prostitute, and the narrative develops farcical overtones, with Anthony at one point having to hide her in the wardrobe in his hotel room.

In the end, this novel amounts to more than just a fast-paced comic adventure story, largely because of the very persuasive historical content, but also in its exploration of art, and more particularly Anthony's relationship with his subject Velasquez. Though none of the warring political factions is much concerned about the fate of the Velasquez, it is the one thing Anthony truly cares about, and as the novel develops the painting becomes a symbol for art in general and its role and importance in society. Significantly, its fate is sealed by the maelstrom of historical events which happen around it.

An Englishman In Madrid

Eduardo Mendoza

Quercus, £16.99