Restoration is a period piece that takes its title from the name given to a short and distinctive age. The novel's narrator, Sir Robert Merivel, physician and lecherous would-be courtier, styles himself a representative of this age, whose leader of fashion is King Charles II himself. A sense of period is not a matter of props and costumes, though Tremain's narrator does love his fabrics and furnishings. At the height of his fortunes he salivates over his Withdrawing Room, planning "to hang the walls with ruched vermilion taffeta and Peking scrolls, to upholster my chairs in scarlet and carmine and gold". And he will always tell you what he is wearing.
Nor is it a matter of staging historical events, though Merivel does observe the Great Plague and then the Fire of London. In Restoration, the sense of period comes upon us most strongly in two ways. Firstly it comes through the narrator's language. Tremain has given him a prose that echoes the sentence patterns of the late 17th century, but only distantly, so that it never seems quaint: "Our amours are not of the tearing and clawing kind, but agreeably hot for all that and tolerably frequent." Merivel always strives for a gentlemanly syntax.
Secondly it comes through the characters' sense that their times are distinctive. Period is a matter of characterisation as much as of circumstantial detail. This is why Henry James thought that convincing historical fiction was impossible. You can "multiply little facts that can be got from pictures and documents, relics and prints", but what is out of reach is "the invention, the representation of the old consciousness". Reading her Pepys as she must have done to write this novel (a fact signalled by the diarist's fleeting, unnamed appearance on two occasions), Tremain will have extracted not only the particularities (food, fashion, street signs) but also the writer's awareness of his unprecedented times. This she gives to her own protagonist.
"What has dawned," Merivel tells us early in his narrative, "is the Age of Possibility." He tries to put a name to the period in which he finds himself, for the novelist has realised that "period" is not simply a matter of historical retrospect, it is also a kind of self-consciousness. The leading characters in Restoration are shaped by their awareness of the age through which they find themselves living. "I like you, Merivel," says the king. "You are utterly of our times." Merivel is flattered, though the reader feels less sure that this is a compliment. Merivel has just agreed to marry the king's mistress in order to preserve her respectability and conceal her continuing relationship with him. He has agreed to be a cuckold in return for a pleasant country house: he is a man of his times because he is happy to be bought.
Tremain makes Merivel a man who resigns himself to the period in which he thrives. His wife asks him if he has mistresses. "'Naturally,' I replied, 'I am a man of my time.'" He makes it sound as if he were obliged to follow the libertine mores of the Restoration. This is in itself a period attitude: if you read the lyrics of the great libertine poet of the period, the Earl of Rochester, you will know that the rational debauchee describes his promiscuity as a kind of compulsion. He is just being true to human nature.
If it is an age whose representatives gratify their appetites, it is therefore also one in which life rewards the ruthless. "But this age is stern, Merivel," the king tells our hero, "and those who are afraid will not survive it." Merivel is to be dismissed from his privileges as a favoured courtier, his house and property taken from him, and will have to survive on his own. Exiled to the madhouse in the fens where he labours to bring comfort to the lunatics, he tells his fellow "keeper" Ambrose: "I was once told I was a man of my time ... now I do not belong to it at all." The madhouse, run by Quakers, is a careful piece of period description on the novelist's part, but it takes the narrator outside his period and into a timeless limbo.
The renunciation and self-sacrifice of these Quakers is a rebellion against the age. Pearce, Merivel's dour, intelligent friend, has undertaken his devotion to the lunatics in "despair at the greed and selfishness of our age which he believed was like a disease or plague, to which hardly any were immune". He tells Merivel that the lunatics in his care are "the only innocents of the age, which itself is a lunatic age, because they are indifferent to glory". Yet even he, with his renunciation of the world, is wedded to his times. He is obsessed with the new knowledge of natural philosophy, and especially medicine and botany.
At the end of the novel, the king, Merivel's patron, assures him that he can enjoy his recovered prosperity in peace and security. Or at least "until my reign ends and another age comes". For the period is given its name by the return of absolute monarchy to England after a time of revolution, and when this monarch dies it will be a different age. Period is political reality too.
• John Mullan is professor of English at University College London. Join him and Rose Tremain for a discussion at 7pm on Wednesday 26 September at Hall Two, Kings Place, 90 York Way, London N1 9AG. Tickets: £9.50/£11.50. Box office: 020 7520 1490. www.kingsplace.co.uk
