Monstrous Regiment: Book 3 in the Industrial Revolution arc.
Industrial Revolution - Pretty much what it says on the tin. Each of these books brings a new technology to Ankh-Morpork which thereafter feature strongly in the following books. While other arcs focus on character development, these arcs pull Ankh-Morpork into the 19th century.
'Trousers. That's the secret...Put on trousers and the world changes. We walk different. We act different. I see these girls and I think: idiots! Get yourself some trousers!'
Women belong in the kitchen - everyone knows that. Not in jobs, pubs or indeed trousers, and certainly not on the front line. Polly Perks has to become a boy in a hurry if she wants to find her brother in the army. Cutting off her hair and wearing the trousers is easy. Learning to fart and belch in public and walk like an ape takes more time. And there’s a war on. There's always a war on. Polly and her fellow raw recruits are suddenly in the thick of it.
All they have on their side is the most artful sergeant in the army and a vampire with a lust for coffee. Well . . . they have the Secret. And it’s time to make a stand.
The bulk of Monstrous Regiment takes place in the small, bellicose country of Borogravia, a highly conservative country, whose people live according to the increasingly strange (and harmful) decrees of its favored deity, Nuggan. The main feature of his religion is the Abominations; a long, often-updated list of banned things. To put this in perspective, these things include garlic, cats, the smell of beets, people with ginger hair, shirts with six buttons, anyone shorter than three feet (including children and babies), sneezing, rocks, ears, jigsaw puzzles, chocolate (which was once Borogravia's staple export, plunging the country into increasing poverty), and the colour blue.
The list of "Abominations Unto Nuggan" often causes conflicts with Borogravia's neighbours, and the uncertain whereabouts of Nuggan leads the inhabitants of Borogravia to deify their Duchess, to whom they pray instead. This slowly causes problems as, on the Discworld, belief grants power. Borogravia is in the midst of a war against an alliance of neighbouring countries, caused by a border dispute with the country of Zlobenia. Rumour is that the war is going poorly for Borogravia, though the country's leadership (and "everyone") denies it.
Polly Perks's brother Paul is missing in action after fighting in the Borogravian army. Paul is naive and believes what he is told regardless of the credibility or political leanings of the source, and even though Polly is more qualified to take over the family business (a famous pub known as "The Duchess"), according to Nugganitic law, women cannot own property, so if Paul does not return the pub will be lost to their drunken cousin when their father dies. Partly to ensure her own future but mainly to ascertain whether Paul is alive, Polly sets off to join the army in order to find him. Women joining the army are regarded as an Abomination Unto Nuggan, so Polly decides to dress up as a man (another Abomination) and enlists as Private Oliver Perks (taking her name from the folk song Sweet Polly Oliver) a song that her father sang to her when she was a young girl.
While signing up, Polly encounters the repulsively patriotic Corporal Strappi, and the corpulent Sergeant Jackrum. Despite her apprehensions regarding Strappi, she kisses the Duchess - that is, she kisses a painting of said noble - and by doing so, joins up. Due to the shortage of troops, her fellow soldiers include a vampire named Maladict, a Troll named Carborundum, and an Igor named Igor. They also include "Tonker" Halter, "Shufti" Manickle, "Wazzer" Goom, and "Lofty" Tewt.
That night, Polly encounters an unknown supporter whilst answering a call of nature, who assures her that although he or she knows that Polly is a girl, they won't give her away. They also gives her some hints on how not to be discovered. Over the next few days, Polly makes a startling discovery: Lofty is also a girl. Since Lofty and Tonker are always together, Polly assumes that Lofty joined the army to follow her man, just like in "Sweet Polly Oliver". Later, she finds out that Shufti is another girl, and a pregnant one. She also joined the army to find her man; in this case, the father of her child, who she'd only known for a few days, and is known as Johnny.
Gradually, Polly discovers not only that everyone in her regiment, except Maladict, is female, but also receives confirmation of Borogravia's bleak situation. Most of her country's forces are captured or on the run, and food supplies are limited. This point is driven home when Igor (actually Igorina) demonstrates her surgery talents and saves several lives among a group of badly injured fleeing soldiers.
The regiment, under the leadership of their inexperienced commanding officer Lieutenant Blouse, makes its way toward the Keep where the enemy is based. Meanwhile, thanks to a chance encounter where the regiment unknowingly subdue and humiliate an elite enemy detachment, including Zlobenia's Prince Heinrich, their exploits become known to the outside world through William de Worde and his newspaper. Their progress particularly piques the interest of Commander Vimes, who is stationed with the alliance at the Keep. Vimes has his officers keep track of the regiment, occasionally secretly providing aid.
Polly and most of the regiment are able to infiltrate the Keep, disguising themselves as washerwomen, and once inside plot to release the captured Borogravian troops. They manage to do so, and Borogravia is able to retake much of the Keep, but when Polly admits they are women, their own forces remove them from the conflict and they are brought in front of a council of senior officers, where their fate will be decided. With the council about to discharge them and force them to return home, Jackrum barges in and intervenes, revealing that several of the military's top officers are actually women as well. But in the midst of this revelation, the Duchess, now raised to the level of a small deity by Borogravia's belief, takes brief possession of Wazzer, her most passionate believer. The Duchess urges all of the generals to quit the war and return home, to repair their country, returning their kiss of service, and ending their obligation to her.
In a private moment with Jackrum, Polly reveals to her Sergeant that she now knows "him" to in fact be a woman. The regiment is sent to the enemy and successfully negotiates a truce, and military rules are changed so that women are allowed to serve openly and Maladict reveals himself as really being Maladicta. Polly finds her brother alive and well and they return home to The Duchess with Shufti, who joins Polly in her refusal to be subjugated on the basis of her gender and marital status. The other members of the regiment go on to lives that they would not have been able to consider before their emancipation, Igorina opening a gynaecology clinic on the basis that many women would prefer to see a female practitioner.
Sometime later, despite the peace they had desperately fought for, conflict breaks out again. Polly, having received correspondence from Sgt Jackrum, leaves the tavern to seek new ways to fight a war using the influence she gained and finds herself in the role of commander of boy-impersonating females who are marching off to war.