The title was inspired by previous statements in the first two books. The words 'War makes monsters out of men' were conceived by Ben in the Knife of Never Letting Go, and have since been echoed by Todd, Viola and Mayor Prentiss.
The book picks up where the previous one left off. A Spackle army marches on New Prentisstown from one direction, and the forces of the Answer from the other. Mayor Prentiss has been freed by Todd to help defend the city, whilst Viola is trapped with the Answer and the scout ship, in hopes of acquiring additional aid.
The Mayor assembles his army and orders attacks on the Spackle army at the waterfall near the city. The Spackle, however, while outgunned, are thousands in number, and their weaponry has evolved since the first war thirteen years prior. While keeping the bows and arrows, a new weapon that unleashes white 'sticky' fire is mounted on the back of their bullock -like creatures, labelled ‘battlemores’ by the settlers. This, along with another weapon - white sticks that shoot a mixture of organic chemicals - easily beat back the Mayor's first wave. Todd is nearly obliterated by the fire weapon, but uses his Noise in the same way he used it against the Mayor to incapacitate the rider of the battlemore. A second wave of soldiers, this time utilising artillery to cover them, are able to send the Spackle back in retreat.
Meanwhile, Viola finds Mistress Coyle with the scout ship, which has landed at the base of the demolished communications tower. Two friends from the settler convey, Simone and Bradley have been sent to look for Viola, and are shocked at the state she, and the settlement, is in. Mistress Coyle, at first surprised that Viola is alive, quickly recovers, and tries to explain her actions. But Viola silences her and warns Simone and Bradley not to trust the deceptive healer.
Then, concerned with Todd’s safety, Viola asks Bradley to launch a survey probe to scope out the scene. After seeing the war and Spackle weaponry, and faced with the risk of losing Todd, Viola nearly launches one of the scout ship's point-to-point missiles at the battle. However, the Mayor’s secret artillery is fired first. Bradley and Simone accept Viola's decision that nobody is trustworthy, save for Todd, and send the probe to observe the Spackle encampment, revealing the enormous numbers they have.
The narrative then switches to a Spackle in the midst of the Spackles’ camp. He is an outsider, the Return, and exists as a separate entity to the Land, the Spackle. He is the only surviving Burden, and seeks revenge on the Clearing for killing his ‘one’. His viewpoint ends with Bradley’s probe being shot down.
The next day, Spackle dam off the river far down, blocking the water supply, and attack the Mayor's camp with a bow-launched burning boomerang that is capable of returning after hitting several targets and then being re-launched in a matter of seconds. Coyle attempts to manipulate Viola to launch a missile to save ‘her boy’, but Bradley is heavily against it. Todd, being watched by a new probe, becomes a stationary target when his horse, Angharrad, refuses to move and he refuses to leave her (he cannot do the same thing he did to Manchee). When Viola thinks she sees Todd getting hit by one boomerang, though it was actually the Mayor's horse, she launches the missile, which causes a catastrophic explosion. This kills all the Spackle archers and blocks the way up and down the hillside, separating the two armies.
The Mayor then sends the thought ‘I am the circle and the circle is me’ into Todd, softly, though Todd objects and threatens to hurt him. Then he notices that this quietens his Noise, and pushes away the memories of war, and so he finds himself repeating it for comfort and power. Viola arrives at the town square, shaken that she had caused a war for Todd. Todd tells her that he would have done the same thing, but she finds little comfort. She then notices that Todd’s Noise is quieter, and he sees that she is ill, but the two do not elaborate on either conditions.
The narrative switches again to the Return, who is arguing with the Sky, the leader of the Spackle, and demands more action and attacks. The Sky refuses to do anything that will not be for the good of the Land, but asks the Return to trust him, and takes his to the Pathway’s End. He reveals ‘the Source’, a human captured before the war began. The Sky offers the Source to the Return if the two armies reach peace. The Return is unsure why such a person would be considered a reward until he recognises the Source as Ben, Todd adoptive father.
Meanwhile, refugees begin to pour in from the outskirts of New Prentisstown - most of the men joining the Mayor's forces in the town square, while most of the women joining Coyle's forces at the landing site. The Spackle begin attacking the town at random, destroying various targets and killing any scout parties the Mayor sends. The white stick weapons are realised to be some sort of ballistic weapon that shoot acid, which vaporizes on leaving the barrel, but keeps cohesion until it hits a target. Eventually, the water store is attacked, and nearly all of the remaining water is lost.
During this time, the Mayor loses control over ordering soldiers with his Noise, and Todd hits him and takes over. The Mayor praises him for having such ability, but Todd is preoccupided with the discovery of James’ body (a soldier who feeds the horses, drowned in the water tank flood). He would not have been killed if Todd had not controlled him to go get extra water for Angharrd, and this causes Todd to regret having controlled him for his own desires.
Now without water, and the Answer without food, the two parties are forced into a peace talk at the destroyed House of Healing. The Mayor and Todd meet Coyle, Simone, Bradley and Lee, and Todd and Viola quickly disclose the fact that they have brought additional soldiers who are in hiding. However, Todd doesn’t reveal why his Noise is becoming harder to hear, and Viola refuses to acknowledge that the infection from her arm band is starting to seriously affect her.
After negotiations, a peace is reached and a transfer of food is made from New Prentisstown to Coyle's camp. Now that the two groups are working together, the Mayor lures Spackle out to fight them, and the Answer provides bombs to destroy the attacking Spackle. Bradley, nicknamed ‘the Humanitarian’, angrily convinces Todd and Viola to stop this. A Spackle is captured by the Mayor, and then sent back to the encampment with two messages. The Mayor sends one of absolute silence, and Todd tells them that they want peace. He hears the Spackle call him ‘the Knife’ before running away.
Angry that the Mayor undermined her, Mistress Coyle sends a bomb into the Spackle stronghold. The Spackle respond by sending the same Spackle that was captured, telling them to send two people to meet the Sky on the hill tomorrow morning.
Viola and Bradley, who has caught the Noise germ and will thus appear more trustworthy, are sent to talk. The Sky greets them, but one individual - 1017, the survivor of the Mayor's genocide, and the Spackle who had sworn vengeance on Todd when he was saved - recognizes Viola as Todd's ‘one’, and attempts to murder her. However, he stops when he sees the ID band that had been banned onto her wrist, and in his surprise is taken away by the Land. Hesitantly, the peace talks resume.
That night, however, the Spackle launch a surprise attack on the Mayor and Todd. But the Mayor, who had read the Sky’s Noise through the messenger, knew of this plan beforehand and had already set up his artillery and soldiers. After killing the attacking Spackle, the Mayor grabs Todd’s comm and tells the Sky that he can read him and that he should forfeit. The Sky, shocked, agrees.
The Return, meanwhile, has returned to the Pathway’s End to kill Ben as peace has now been reached. The Sky meets him there, and watches as the Return fails to murder Ben, unable to kill just like Todd. He resents this, but the Sky tells him he will need this knowledge when he becomes the Sky, and then wakes up Ben, ignoring the Return’s confusion at this statement. When Ben wakes, he can speak in the language of the Land.
The next day, peace is reached, though negotiations are scheduled to continue. The Mayor strategically announces that they have beaten the Spackle to the hilltop campers, and that a cure for the ID band infection has been found. Mistress Coyle is outraged and convinced that the Mayor planned this, and takes to testing the cure, warning that audience that the Mayor is up to something. Her accusations are met with booing and anger, and she recedes in bitter anger. Soon, testing reveals that the cure uses an aggressive antibiotic mixed with an aloe the Mayor claims he found in Spackle weaponry, which allows the medicine to disperse ten to fifteen times faster. Viola asks Todd to decide if she should take the cure.
At the town square, the Mayor teaches Todd how to read by giving his skill of reading through Noise. After reading his mother’s diary, Todd asks the Mayor if the cure is real, and the Mayor simply says it will. Convinced, Todd brings the bandages to Viola the next morning to treat her infection.
A little later, Mistress Coyle delivers a speech on her intentions to resign and hand over leadership to the Mayor fully. However, it quickly turns out to be a plot to kill the Mayor using a bomb strapped to her own person. Todd, in an attempt to save Simone who is in front of Mistress Coyle, inadvertently saves the Mayor instead, foiling her plan and revealing that he can control people with his Noise (as he ordered Wilf to jump from his position near Mistress Coyle). The Mayor thanks Todd endlessly but Todd is horrified that he has saved the Mayor subconsciously and not Simone, as proven by the replay on nearby probes.
Viola is similarly horrified, unsure of who Todd has become. She tells him that she hates his silence, and they fight, Todd convinced that the Mayor has changed and Viola accusing him of turning into the Mayor. She then tells him that she cannot trust him anymore; that he isn’t him anymore, causing his Noise to flare up in anger, shock and hurt. The Mayor interrupts before more can be said, intent on delivering another speech to show the citizens that he is okay.
As the Mayor starts the speech, and asks Todd to be his son, Ben and the Return arrive. Todd, overwhelmed by happiness, launches himself at Ben and leaves the Mayor on stage, ordering people to get out of his way as he runs. The two embrace, and the Mayor is left feeling betrayed and unwanted. Now that Ben can speak the Spackle’s language, he prefers using his Noise to communicate, and, consequently, Todd’s noise opens up around Ben as they talk. After this, Todd asks Viola to leave New Prentisstown with him when the war is over, and declares that he doesn’t care about what will happen to everyone without his supervision. His Noise is audible once more, and he openly thinks about how beautiful Viola is and how he wants to hold her. He hastily apologises, but Viola kisses him, and thinks that it feels like ‘finally’.
The present party splits to settle a peace immediately, leaving Todd and the Mayor alone. Angered that Todd no longer cares about him, and wishes to leave him the moment peace is established, the Mayor shoots a nearby Ivan and steals the scout ship, with an unconscious Todd on board. Learning how to operate the scout ship from Bradley’s Noise, he launches the missiles at the Spackle, where Bradley, Viola and Ben are still negotiating terms. Todd wakes up to this, and watches helplessly as the missiles descend. However, nothing detonates. The 'hooper' bombs - cluster bombs launched directly from underneath the ship - also fail. It appears that Simone had defused them before she died.
The Mayor decides to try something different, remembering a trick Bradley’s grandfather had taught his grandson. He remixes the fuel and releases it. The fuel mixture is the same used by Viola's campfire box in The Knife of Never Letting Go - only a few drops was sufficient to blow up the bridge between old Prentisstown and Farbranch. The fire kills many Spackle, including the Sky, and the Return is nominated by the Sky to become the new Sky, and takes his place. Todd watches on board and tries to call Viola on her comm, shaping his Noise in such a way that it resembles how it was when the Mayor taught him how to read. He then manages to use the manual on the ship’s screen to call Viola and tell her they are headed for the ocean, before being cut off by the Mayor.
The water in the dammed river is released, and Viola and Bradley ride to warn all citizens to evacuate the town.
The Mayor then lands at the ocean. The new Sky decides to attack the settlers, while the Mayor had ordered his remaining captains to attack the Answer and march into the town square to die. However, Lee, using a rifle with Wilf's help, kills Captain Tate and the soldiers join the remaining people. Wilf and Ben manage to dissuade the Sky from the attack, Wilf’s honest and open Noise showing the Sky how to listen to the Land, leaving the Mayor disappointed.
Viola and Bradley now ride to the ocean in hopes of aiding Todd. However, Acorn, Viola's horse, dies from exhaustion, and she is forced to ride Angharrad while Bradley is left behind. At the ocean, Todd and the Mayor are in a Noise battle - each launching their Noise at the other. When Viola arrives, this gives Todd new strength and he is able to overcome the Mayor. The Mayor congratulates Todd, then says he will be a fine leader of New World - a world he doesn't want to be a part of. He walks into the ocean, where a killer fish takes him.
The Sky arrives on his battlemore with Ben. He sees Viola and Todd, but mistakes Todd for the Mayor and shoots him in the chest with the acid rifle. Todd’s Noise disappears, and he dies, driving Viola to threaten shooting the Sky back. The Sky, revealing that he shot despite knowing that it might have been Todd, realises how wrong he was to do so, and feels the regret Todd felt when killing a Spackle in book one. Through his Noise, he shows Viola how to use the white stick weapon, but she stops short of killing him, realising that it would cause never-ending war, and that no one would remember Todd, and everything that he had done, after. She warns the Sky to get out of her sight, and returns to Todd. Ben suddenly asks if she can hear anything, swearing that he can hear Todd’s Noise.
The Spackle attempt to cure Todd with their medicine, and house him in the Pathway’s End. The remedies are working, and his Noise returns in burst on and off. The main convoy is about to arrive to a large ceremony, one which Viola will not attend because she will not leave Todd’s side until he wakes up, though she knows that he will be changed, like Ben was. The Sky has promised not to take the cure for the band infection until Todd is cured, in an act of self-discipline, but Viola does not forgive him and will not let him enter to see Todd. Everyday, Viola continues to read Todd's mother's journal to him, hoping that he will hear and come back.
The epilogue cycles through Todd's experiences in the coma. He is entering his old memories, at his school, at Farbranch, but also human and Spackle memories from all over New World. He searches for Viola, unsure who she is, who he is. Every now and then, he hears abstracts from his mother’s diary, and Viola, and he begs Viola to keep calling for him, because she’s saving him, and because he’ll keep searching. The novel ends with hope that he’ll return, the last lines being "Keep calling for me Viola-, Cuz here I come".
Critical reception has been largely positive. The Bookseller responded by saying that the novel was 'breathtaking” and noted that it is “innovative, intense writing at its incendiary best'. 3 Am Magazine has praised Ness and compared him to Philip Pullman, Robert Cormier and Paul Zindel, labelling Chaos Walking to be “too good for the Young Adult strapline.” Sally Morris from the Daily Mail went on to say "the ending is superbly well handled and Ness brings this original series to a close with the high-level tension and ambiguity he has maintained throughout."
In the UK Sunday Times, Amanda Craig compared Monsters of Men with Avatar (2009 film), commenting that though it "lacks the visual beauty of Avatar, or its comforting clichés", it has breakneck adventure and the novel is "one of the most interesting fantasies" ever published.