Where's the Left Arm?

The chapter opens in the dark of a Titan's gut. Eren, his left arm bitten away before he was swallowed, drifts among corpses and the dying, watching a fading soldier murmur for her mother as she sinks into the bile. Grief hardens into a renewed oath to wipe every Titan from the earth, and that fury takes physical shape. A colossal arm forces its way up the creature's throat, the beast collapses, and Eren tears free of its body inside a giant frame of his own. His first act in this form is to meet a charging Titan and drive a fist clean through its weak point, and the ease of the kill only sharpens his hunger to destroy more of them.

He comes to in human form muttering about killing them all, and finds blades and artillery pointed his way. Kitz Weilman commands the Garrison cordon and treats the boy who walked out of a Titan as humanity's enemy, demanding a straight answer: man or Titan? Rico Brzenska advises executing him before he can change again, while Ian Dietrich cautions that cutting down Mikasa, who stands guard beside Armin, would rob the military of one of its finest. Eren's answer, that he is human, buys nothing; Weilman signals the guns anyway. Mikasa scans for a route up the Wall while Armin gropes for words that might sway the soldiers. In the stretched second before impact, Eren relives a buried memory of his father Grisha pressing a key into his hand, administering an injection that will cost him his memories, and pointing him toward the basement where answers wait. Told that controlling this power is the only way to protect Mikasa, Armin, and everyone else, Eren bites into his hand. When the smoke clears, a half-formed Titan torso crouches over his friends, the cannonball stopped cold.

Eren consciously wields the power of the Titans for the first time, escaping a Titan's stomach and killing another Titan with a single punch through its weak spot.

Kitz Weilman puts Eren under interrogation and declares that he and the friends defending him will be executed as threats to humanity.

A flashback shows Grisha Yeager entrusting Eren with the basement key and injecting him with a substance tied to his new abilities, at the cost of his memories.

Eren's partial transformation deflects the incoming cannonball, closing the chapter on the skeletal torso shielding Mikasa and Armin.

Japanese publication came on June 9, 2010, with the English release following on December 4, 2012. The chapter opens volume 3, belongs to the Wall Sealing arc, and sits between The Beating of a Heart Can Be Heard and Response, with the side story Captain Levi also listed before it.

The anime covers these events in its ninth episode, Whereabouts of His Left Arm: The Struggle for Trost, Part 5, whose similar title is the reason the two pages carry a disambiguation notice.