After Ling explains to Alphonse and Winry the reasoning behind Major Armstrong's abduction of Edward as well as Roy Mustang's plan for smoking out the Homunculi, he notices Lan Fan's signal flare in the distant sky and informs them that the light they see is the "signal to strike". Alphonse, considering this mission vital to discovering the identity of Maes Hughes' murderer, decides to go along, promising Winry that he will return safely and relate to her everything that happens.
Deep in the Homunculi's underground lair, Lust informs her Father that the situation involving Number 66 will soon be cleanly resolved, as she has sent Envy and Gluttony to hunt the fugitive down. However, her trust in their ability seems unfounded, as Envy struggles against Lan Fan's combat prowss, having been "killed" three times at her hand. Envy attempts to blend into the crowd for a sneak attack, but is effortlessly found out by the Xingese bellatrix's ability to sense the Homunculus' inhuman presence. In the tower, Lt. Hawkeye is equally distressed by Gluttony's strength and apparent immortality, having fired several shots directly into his head with nearly no effect. But when her handgun ammunition runs out and the monster prepares to eat her alive, she is rescued by the timely interventions of Black Hayate and Kain Fuery who tosses Hawkeye one of his pistols before both officers open fire on the enemy, again, to no avail. Fortunately, Colonel Mustang appears in the stairwell behind them and blasts Gluttony out the window with Flame Alchemy, sending him plummeting to the ground on top of Envy. Envy, fearing his opponents' obvious battle strength, attempts to flee the scene by donning the guise of a dog, but his escape route is cut off when he accidentally runs into Al and Ling, who are shocked to see the dog morph to human form when Lan Fan resumes her attack. As Alphonse notices the Ouroboros mark on Envy's left thigh, Gluttony arrives as backup and Lan Fan informs her prince that these strangely invincible foes may hold the secret to immortality that they seek.
In the tower, Riza scolds Roy for coming to the front line and revealing himself to the enemy, but her ire at his disregard of protocol is cut off when Fuery informs the two that their quarry is on the move again. Ordering Fuery and Black Hayate to stay behind and remove all traces of the operation, Mustang and Hawkeye head to their car to rendezvous with Havoc, who reports to them that Barry has already set out after his errant body. Alphonse appears on the scene as well and the four of them follow Number 66's trail, hoping that the fleeing assailant will lead them directly to the enemy stronghold. Roy catches up with Barry, who confirms that his soul will be able to track his body no matter where it goes. Alphonse informs the colonel that he believes their enemies to be Homunculi, citing his encounter with Greed as evidence, but Roy is skeptical that such monsters can exist until Riza confirms Al's claim by stating that Gluttony did not die even after being shot multiple times in his vital areas.
Their chase leads them to Central Alchemical Laboratory #3, where Roy decides that even this much reconnaissance gives them enough evidence to surmise that their enemy is linked directly to Central Command and orders everyone to pull back. Unfortunately, Barry rushes haphazardly inside after his human body against orders, but Roy decides to play off of the confusion and orders everyone in after him, misinforming the staff inside that they are pursuing a dangerous fugitive and that backup is already on its way. Following the Barrys' destructive swath to a sealed off basement level of the facility, the group decides to split up in order to find their quarry, Al with Riza and Roy with Jean.
Mustang and Havoc reach a series of underground laboratories which appear to have been abandoned and Jean comments that the labs stink of violence while Roy notes that the strange bloodstains on the floor as well as operating tables fitted with leather straps suggest that the research being performed here could not have been legitimate. Their examination of the facility is cut short, however, when Lust arrives, lamenting Envy and Gluttony's inability to keep them at bay. Jean is shocked to find his new girlfriend in such a place, but understands the situation when Roy points out her Ouroboros mark. After Jean assures the colonel that he had known better than to divulge sensitive information to a civilian, Roy demands to know if Lust was involved with Hughes' death and, when she responds that she was involved, but is not Maes' killer, Mustang shoots her in the leg and commands her to kneel for interrogation. Unfortunately, Lust merely regenerates from the wound and informs the colonel coolly that it will take far more than that to make her kneel, prompting the Flame Alchemist to open fire into her heart and head. Lust heals these wounds as well, confirming for the two men that she is, indeed, a Homunculus like Al had claimed. She opens the shrinking wound in her breast to show that, as a Homunculus, she has a Philosopher's Stone at her core which powers her regeneration but when Roy calls her a monster, she states that her thoughts, emotions and physical makeup are identical to those of humans.
Before Mustang and Havoc can make their move, Lust goes on the offensive, extending her bladed fingernails and disabling the Colonel's sidearm as well as rupturing an overhead water pipe, dousing both soldiers and eliminating Roy's Flame Alchemy as a threat. With the tables overturned, Roy and Jean flee the room, but Roy reveals that he has yet another trick up his sleeve; with the Transmutation Circle on the back of his glove and a cigarette lighter borrowed from Havoc, he transmutes the water filling the room into combustible hydrogen and flammable oxygen which he then ignites, incinerating Lust in a makeshift furnace. Mustang and Havoc cautiously reenter the devastated room to investigate and though the Colonel comments that the Homunculus was definitely incinerated, he warns Havoc to be on alert for Lust's regenerative abilities. Unfortunately, at that very moment, the Homunculus' bladed talons shoot out from the rubble, impaling the 2nd Lieutenant through the abdomen and causing him to collapse. Lust mocks the two as she regenerates her body, claiming that there is no way for Roy to save Jean from death by exsanguination, but the Flame Alchemist disagrees, plunging his hand into her chest to rip out the Philosopher's Stone, with which he hopes to augment his rudimentary bio-alchemy knowledge enough to heal Jean's wounds. Without its supernatural core, Lust's body crumbles to dust, but quickly re-forms around the Philosopher's Stone in Roy's hand and she stabs the Colonel through the abdomen with her bladed fingers, reprimanding him for being so rude as to grope a lady's breast.
- Hiromu Arakawa already had given a foreshadowing about the fact Mustang Unit be personification of chess pieces when she drew the cover of this chapter, since there is a chessboard background behind the characters.
- In this chapter, it is first depicted that Envy's mass is far greater than it seems, as he crushes a metal railing as he lands on it.
- In this chapter, it is revealed that 2nd Lt. Havoc has a weakness for women with large breasts.
- Havoc states that the cigarette lighter he holds is a gift from his ex-girlfriend.
- In the "extras" section of volume 10, Hiromu Arakawa is having a conversation over the phone with her editor, discussing that they should cut back on the comedy in this chapter.