THE final part of Louise Welsh’s Plague Times trilogy starts seven years after the outbreak of the Sweats. The flu-like pandemic that spread across the world has abated, and the survivors are left to either rebuild society or ensure its continued disarray. Stevie Flint, the main character in A Lovely Way to Burn, and Magnus McFall, from Death is a Welcome Guest, have escaped the chaos of London and found a new home in the Orkney Islands. They are both tired of surviving and want to regenerate some semblance of normalcy and democracy. During the last few years, however, Stevie “discovered a talent for violence” and behind her peace-loving exterior she is “secretly longing for some danger to make life seem important again”.

This, in a sense, is the crux of the whole trilogy. Although the invisibility of the virus adds to its menace, the Sweats could be replaced by any mass disaster. What the novels are essentially about is what happens to human psychology during societal collapse. In No Dominion, the relative calm of Orkney’s community is disrupted when three new survivors arrive in the middle of a new election. Nobody quite knows how the young people on the island have been affected by the violence of their upbringing. But soon it becomes clear. A group of teenagers, led by Magnus’ adopted son and his girlfriend, run off to the mainland with the newcomers, stealing a young baby and leaving two islanders dead. The young are heading for the bright lights of Glasgow, unaware of the extent to which Scotland has become a clan-like warrior state. Stevie and Magnus set off in pursuit, hoping to save the children from themselves.

Welsh’s narrative is a strange convoluted one — sometimes dizzying in its whimsy — and can seem like a way for Stevie and Magnus to encounter as much brutality as possible. This is not a novel for the weak-hearted. There are abattoirs of blood and more violence than in a Hammer horror movie. Her previous novels, like Girl on the Stairs for example, played with elements of the gothic. In No Dominion, there is an absurdist pulp fiction quality to the drama. At the end of one chapter, Stevie and Magnus break into a fuel storage facility. They encounter “a cadaver-thin man, with a battery torch in one hand and an axe in the other. The man said, ‘Welcome to the funhouse.’” This kitsch horror will be fun for some readers, but plain silliness for others.

Most people will know not turn to Welsh for high style, and there are few linguistic pyrotechnics anywhere in this trilogy. The prose is often utilitarian, the accustomed artistic mode for modern thrillers. Welsh can write excellent slow atmospheric scenes, and when she does her fictional landscape shines with clarity. But she can also pack her sentences with too much sentiment and tepid humour, especially in the dialogue. Overall, however, her style is simple and clean enough not to hinder the reader as they race through the pages. No Dominion is fast-paced and unrelenting. It’s not a novel for re-reading, but it’s an enjoyable novel to read.

If her prose doesn’t astonish, her ambition does. The three novels together come in at over one thousand pages, and even the sub-plots are complex – she explores the healthcare system in the first novel, the justice system and religious fanaticism in the second, and the formation of political superstructures in the third. Stevie’s contender for the presidency of Orkney is a snide trader called Bjarne, who dies early in the book. As the chase moves to Glasgow, it becomes clear that, before his departure, Bjarne tried to make some unsavoury deals to improve Orkney’s resources. All political systems need labour, and in desperate times, the physically fit become a valuable commodity. It turns out the hopeful fun-loving teenagers have been dragged into a personal and political battle for the launch of a new urban regeneration project spearheaded by a provost called Bream.

This imaginative reach is Welsh’s greatest achievement. Her sociological insights encompass the extremes and subtleties of human behaviour. Few thriller writers would bother to explore in much detail the diversity of human (and non-human) responses to a pandemic and its aftermath. Too often these kinds of novels become a way for a minority of goody goodies to show how good they are in the face of evil. But all the characters in Welsh’s trilogy have corrupted themselves, even if most of them don’t want to “talk about the things they had done to survive whilst everyone around them died.” In this respect, Stevie, in facing up to this uncomfortable truth, is the most realised and enduring character.

At the beginning, she remembers arriving on Orkney, helping to “fumigate” the island and dispose of hundreds of bodies, those who “had either died of the Sweats, or taken the other way out…” Suicide lingers at the margins of No Dominion: a way to defy a fate worse than death and as a desperate shot at beating death at its own game. Some novelists are drawn to life; others to death. It’s clear on what side of the divide Welsh stands. At the end of this alarming trilogy we are left to wonder if life, in its brutal extremes, is as sacred as we like to think it is.

No Dominion by Louise Welsh is published by John Murray, priced £16.99