The Fifth Laboratory

At the shuttered Laboratory 5, Alphonse is set upon outside by an armored stranger who names himself Number 66, while Edward slips inside and finds a vast chamber marked with a bloodstained five-point array. A second guardian, Number 48, steps from the dark and declares he must cut down any trespasser. The two trade blows until Ed catches the same empty ring he hears whenever he spars with his brother, prompting the guard to admit he too is a soul bound to armor. Number 48 introduces himself as the executed serial killer known as Slicer and even exposes the blood rune inside his helmet, offering Ed victory if the boy can destroy it.

Ed's newer automail, weaker than the old model, begins to fail him mid-fight, and he learns from Number 48 that a companion is battling his own partner outside. There, Alphonse strips off Number 66's helmet and finds him just as hollow, only for the killer, calling himself Barry the Chopper, to plant a poisonous idea: that Al's memories and very soul may be a fabrication invented by the alchemist he calls his brother. Unable to prove he was ever human, Al starts to doubt his own existence. Inside, Ed severs the helmet from the body, but the headless armor rises, and Number 48 reveals the Slicer was in fact a pair of murdering brothers sharing one shell, each with a separate rune. Cornered, Ed mimics Scar's destruction alchemy and blasts the younger brother apart, then spares them, insisting he judges them human because he considers his own brother human.

Just as the elder Slicer agrees to share what he knows about the Philosopher's Stone, Lust and Envy appear and destroy both killers to shield their so-called Human Sacrifice. Ed's arm gives out and Envy knocks him unconscious. Ross and Brosh arrive to arrest Number 66 as the lab collapses, and Envy carries the wounded boy out before vanishing. Next door in Central Prison, Solf J. Kimblee, transmutation circles inked on both cuffed hands, thrills at the sound of high explosives. On the phone, Hughes urges a soon-to-be-promoted Mustang to gather allies and, jokingly, to marry.

The installment adapts Chapters 11 through 14: The Two Guardians, The Definition of Human, Fullmetal Body, and An Only Child's Feelings. The five-point circle from Laboratory 5 appears on the title card. Ed's shoulder trouble traces back to the A-08 bolt Winry forgot to fit two episodes earlier. Unlike the manga, Envy is shown smashing a large opening through the wall to flee, likely drawing on the enormous mass hidden within his body.