The Night Watch is Sarah Waters's fourth novel and something of a departure from her usual area of historical fiction. Anyone expecting another costume romp full of twists and turns and titillating erotic interludes will be sorely disappointed. Waters first three novels (Tipping the Velvet, Affinity and Fingersmith) are all set in the Victorian era. Night watch brings the lesbian action forwards and sets it in the 1940's during and after World War II. With the change of setting comes a change in mood: Night Watch is a sober, introspective read. One element alone remains the same: the writers stated aim to put lesbian love at the heart of history.

The novel's most striking aspect is how, in this book, Waters makes a serious stylistic departure from previous work in that she chooses to re-tell her story by reversing events. We first meet the main characters in 1947, a group of individuals drained by the war and exhausted by personal dramas. The book then proceeds through two sections (1944 and 1941) to dig back into their pasts to uncover not simply what happened but why. For many people this device has proved too 'tricksy' and reviews often suggest the book's full impact can only be gained from a careful re-reading. This may do both the book and the writer a disservice.

Waters has mentioned in interviews how as a child she had a passion for archaeology and we see here how by re-ordering events, reversing the chronology we engage with the process of uncovering history, digging backwards into the past, like archaeologists who brush away the dust to reveal new and surprising finds. The work of the reader is to uncover unexpected elements from the past, which inform and reshape the present. Kay, a young lesbian who has enjoyed the war as a release, a time allowing her to dress in suits and act gallantly in masculine ways, says about entering cinemas halfway through a film to watch the ending first, "I almost prefer them that way - people's pasts, you know, being so much more interesting than their futures." Waters is asking us to trust the writer to make the past more interesting than the future.

Night Watch revolves around the inter-related lives and loves of five main characters : three lesbians (Helen, Julia and Kay) , a straight woman (Vivien) and a young man imprisoned for attempted suicide (Duncan). Through Helen, Kay and Julia the book explores how love pursued is often love unreciprocated. Some readers may find these three central characters to be Virginia Woolf/Bloomsbury types, teetering on the edge of caricature. Their introspection and well-bred struggles to own their sexual identity occasionally slip into narcissism: they care too much for themselves and too little for a world at war. That aside Waters usually succeeds in making this corrosive love triangle believable, the characters genuine and their affections real.

In contrast the character of Vivien is a near faultless creation, an ordinary woman whose beauty attracts a married sweetheart (Reggie). Vivien is described as a 'glamour girl' but her struggles with family, work and love are far from glamorous. Her brother (Duncan) has been imprisoned for attempted suicide. She feels a fish out of water at work in the typing pool, a place reserved usually for refined ladies who know not to say 'toilet' when the word 'lavatory' is called for. We see how Vivien is trapped in and yet strives to transcend her class. Her ultimately doomed love life is perhaps the most fully realised relationship in the book taking the reader on a journey from laughter and attraction to pain and resignation.

The fifth member of the central quintet, Duncan is perhaps the most problematic and here it can be argued that Waters' reach exceeds her grasp. Imprisoned in Wormwood Scrubs for a botched suicide attempt the frail young man becomes something of a martyr to man's inhumanity to man, more a symbol than a fully realised character. His homosexuality is confusingly hinted at but left unclear in the reader's mind. Around him swirl some of the book's large themes (wrongful imprisonment, the need for people to hide their true identity, the question of conscientiously objecting to all war) and at times Duncan becomes trapped, the sheer weight of ideas smothering his unhappy life. His relationships with Fraser, a well to do, well-educated 'conchy', Mister Mundy an ex-prison officer and the mildly deranged suicidee Alec are all ultimately disappointing.

Against this can be set the books real strength and that is that Waters, an assiduous researcher, has the ability to deploy her knowledge effectively to evoke both the time (the Forties) and the place (a war ravaged London). We step back through the dialogue and description into a vividly realised past. We enter a world where people say 'Blimey!" "Crikey!" and "Chum!, where men wear "dark blue demob suits already shiny at the elbows" and women are dressed in headscarves, "decorated with faded tanks and spitfires". We enter houses where 'there were yellow, exhausted photographs,' or where 'the walls of the room were done in lincrusta, painted a glossy chocolate brown' . We fall headlong into the blitzed city where a burnt, bomb injured man sits 'with glass on his face 'dainty little glints', a dark world where a dentist, can be an abortionist who does 'this other thing on the side'. We watch workers through Duncan's eyes leaving their menial jobs in a candle factory, rushing home to leave an empty space, a yard 'like a sink with its plug pulled'. We weave through the 'handsome higgledy piggledy red-bricked' streets of London and confront the horror of a woman impaled of railings whilst St Paul's stands tall a symbol of 'elegance and reason and … great beauty'. We become caught up in a visceral world, whether it be the central crisis in Vivien's life where blood falls from her 'thick and dark and knotted as a length of tarry rope' or as we arrive alongside Kay at the blast scene where a 'man's head had rolled out and landed at her feet'. We are convincingly in a world where, as Viv remarks, "We might all be dead tomorrow."

Amidst the constraint of war Waters has effectively recreated a world where paradoxically people felt the possibility of escape. In the wrong hands the notion that war licences excess might become clichιd. But in Water's hands some of her characters do, for a time, transcend their constraints. Helen, Julia and Kay find love in the shadow of the Blitz and Kay, a masculine lesbian, is free to behave with 'gallantry' and manliness. Vivien finds passion with Reggie and Duncan's life may change with the promise of a friendship forged initially through kindness in the bleak harsh world of Wormwood Scrubs.

Ultimately this is a serious book where people who had extraordinary lives must continue to strive to come to terms with their past, must struggle, exhausted, to move towards an unknown and unknowable future. As the first line of the book suggests, in life it may not be possible to see a way ahead but only be possible to understand 'the sort of person you've become'.