Armor is the story of humanity's war against an alien race whose foot soldiers are three-meter-tall insects, referred to in the book as "ants". It is also the story of a research colony on the fringes of human territory which is threatened by pirates. The two sub-plots intersect at the end, with each providing answers and insight into events of the other.
The title refers to the nuclear-powered exoskeletons worn by the soldiers, but also references the emotional armor the protagonists maintain to survive.
The protagonist is Felix, an enlistee who's been given "scout" duty on an alien planet in the seemingly endless Antwar. Little is known of him initially but that he suffers from burnout and refuses to die, even when it seems inevitable. When entering into combat, a persona he calls the "Engine" takes over. The Engine makes him into a ruthless killing machine dedicated to keeping him alive at all costs.
The novel starts with Felix and his company assaulting an inhospitable planet aptly named "Banshee." Using armored infantry suits, soldiers drop onto the planet from starships via a teleportation device called "Transit." The attack goes horribly wrong, as Felix's company is completely wiped out, and the mountain fort they were originally supposed to capture is revealed to be a giant hive. In the aftermath of his first encounter, Felix regroups with the surviving humans. There, he meets another highly skilled scout named Forest, who participated in the fleet wide Armored Olympics against decorated soldier Nathan Kent. It is also revealed that the Ants can lock onto the fleet's Transit beacons and barrage them with missiles fired from the Hive, making retreat impossible.
During a fallback to an elevated bluff, a dying soldier keys all functions on the suit and blows up the nuclear powered suit. Several warriors are killed in the disaster, including Forest. Before dying she confesses to Felix that she always loved Kent. With no options left the warriors resort to attacking the Hive to secure transport back to the ship. Exploiting the Hive's tunnel system, they plan to use a wounded soldier as a bomb to destroy it. Before the attack, Felix meets a soldier named Bolov who tells him that as a scout on his first drop, Felix has a 10% chance of survival.
Felix and his fellow soldiers are ambushed by ants on their way up the hive. During the attack, the soldier they planned on using as a bomb is killed, rendering him useless. In a stroke of bad luck, Bolov is also mortally wounded, grudgingly accepts the task of sacrificing himself and is thrown in the Hive by Felix. The Hive is then destroyed and the survivors are able to retreat back to the ship.
After 19 drops on Banshee, Felix has learned that he is an almost unstoppable warrior. His 20th drop is revealed to be a fleet mission to create humanity's first forward operating base on Banshee. He also learns that Nathan Kent will be dropping with him.
The soldiers successfully create an impregnable fort on Banshee. Felix learns from his inexperienced Commanding Officer, Canada Shoen, that one of the purposes of the fort is to show non-combat officers and journalists what the war is like. Felix is disgusted by this revelation. After spending time with Nathan Kent, he tells him about Forest's death and her true feelings for him. Kent is devastated and attacks Felix in a drunken rage. The fort withstands several assaults from the ants and wears them down almost to complete exhaustion. During a celebration for their victory, ants attack from underground and kill dozens of people, before ultimately being crushed again.
Felix discovers that his 21st drop will be with the affluent monarch of an extra-solar planet, named the Masao. They drop onto an area not infested with ants so that he can see Banshee for himself. However, in a moment of climax, Felix is revealed to be the runaway monarch of an equally wealthy planet called Golden. After a tragic freighter accident that killed his wife, Felix fled his responsibilities as monarch and joined the fleet to suppress the memories of her. The Masao, who is his best friend, begs for Felix to return to Golden. He agrees to leave the military, but still refuses to return to his home planet. Only moments before being teleported onto the ship, Felix notices a Hive in the distance that barrages them with missiles, killing the Masao.
In the ensuing chaos on board the ship, Kent grabs Felix and helps him escape from the fleet before ultimately being killed himself by the ship's trigger-happy security.
Most of the action of the later two-thirds of the book takes place on Sanction, a planet far removed from the fighting, at a Fleet research facility.
Jack Crow is a notorious celebrity and one-time pirate. A morally questionable character with views and opinions that are just as questionable, he is a tough man who does not hesitate to kill. He is constantly at odds with his own morality but he knows the difference between his celebrity reputation and his real personality. At times, his reputation is more of a burden than a blessing.
Jack, having been imprisoned, is successful in an escape attempt, permits him to reach the ship of a mutineer and deserter from the Antwar named Borglyn. Jack strikes a deal where, in exchange for Borglyn saving his life, he will infiltrate and sabotage the Fleet research project on Sanction so Borglyn can access its limitless Fleet power source to refuel his ship, and continue on his deserter way. Borglyn will also pay Jack and give him a small scout ship to go his own way.
On Sanction, Jack takes an old suit of battle armor to project Director Hollis "Holly" Ware, to ingratiate himself and get the necessary access to fulfill his bargain with Borglyn. But Jack is then asked by Holly to participate in an experiment to retrieve the data from the suit's battle recorder, which is the "memory" of the wearer while the suit was active.
Out of curiosity and a bit of false bonhomie, Jack agrees and we see that he, Holly and Lya (a Fleet psychologist and Holly's girlfriend) spend several sessions "immersed" first hand in the life of Felix on Banshee and on board ship. During these sessions, we learn much more of events on Banshee, and are truly introduced for the first time to Felix as he sees himself and experiences battle. A few tidbits of memory also tantalize the researchers (and ourselves) to learn the true identify of Felix.
Through immersions, we learn how extraordinary Felix truly is and how, through either the complacency or gross incompetence of his superiors, he has suffered unbelievably, yet continues to soldier on. This also challenges the trio to deal with the knowledge that the public story of the Antwar is far different from the true horror. These immersions change Jack - as it does the others - so that he chooses to make an almost certain suicidal stand with Holly against Borglyn's attack on the facility.
Throughout the novel, Jack comes into contact with the alcoholic owner of Sanction, a wealthy rancher named Lewis. Lewis is profoundly anti-war and doesn't allow his citizens to carry energy weapons. Jack comes to despise Lewis for his irresponsible drunken nature. However, he later saves Jack and Holly during Borglyn's attack. Lewis is then revealed to be Felix, who dons the black scout armor once more to fend off the attackers. He destroys Borglyn's forces and attempts to attack his ship. At the last second, Borglyn fires all of the ships weapons, seemingly destroying everything around it.
Jack and Holly watch in awe as this happens, and catch a final glimpse of Felix on the back of the starship through a security camera, who appears to be still alive. Felix destroys the camera and is hinted to escape once again. The current rulers of Golden send emissaries to Sanction to search for their legendary protector. The novel ends with Jack, Holly, and Lya starting new lives on Sanction outside of fleet influence.