Where's the Left Arm?

What Happened To The Left Arm is the first chapter of the third volume and the tenth chapter overall.

Summary
The titan, with Eren in its mouth, bites down, removing Eren's left arm. Now in the titan's stomach, Eren is surrounded by the bodies of the dead and dying. In disbelief, he remembers all of his training and how he's changed these past five years. Thinking of everything the titans have cost him, he again swears he will erase them from this world.

Suddenly, a left arm appears and forces its way out of the titan's throat, exiting its mouth. The titan collapses and and Eren, in titan form, bursts out of its abdomen. As he leaves the corpse, he encounters his first titan. Strangely, it reacts to it as it would a human, going in for an attack. With almost no effort, Eren punches a hole through its head, straight through its weak spot. Seeing his new found power, he wants to kill more.

Apparently dreaming, Eren wakes up saying that he wants to "kill 'em dead". Noticing the situation, he sees that soldiers have him backed into a corner, while Mikasas and Armin try to defend him. A couple of soldiers overheard his statement and mistakenly believe he is referring to them. Looking at his left arm, he starts to realize that what he saw wasn't just a dream. An officer reveals the gravity of the situation. The are going to execute him and because they came to his defense, his friends will be as well. The officer questions him asking if he is a human or a titan. Not understanding, Eren asks what he means. Losing patience, the officer states that there were numerous witnesses that saw him coming out of the titan, which in effect, makes him the enemy. Not wanting to risk any more soldiers, an officer near him suggests that they kill him while he's still in human form.

Mikasa informs the surrounding soldiers that if tries to get close to them, she will demonstrate how good she is at what she does. The soldier in charge of the rear guard tell the officer that Mikasa is one of their best and it would be a great loss if she was killed. While Armin suggests fleeing, Eren tries to remember what has happened. When the officer asks again what he is, Eren knows that his answer affects his friends as well. Telling them he's human, the officer orders the cannons to fire on them. Eren tells his friends to leave him as Mikasa looks for a way up the wall and Armin pleads their case to the soldiers.

Eren thinks back to a time with his father. His father hands him a key and tell him the importance of keeping it with him and explains that the injections will cause memory loss. He also reminds him of the basement, saying that although it would be a harsh journey the "power" would be useful and he would find answers there. Time seems to slow down as one last phrase rings through his head, "If you wis to protect Mikasa and Armin, you must embrace this power and become its master". As the cannon fires, Eren bites down on his hand. An explosion seemingly happens around Eren and his friends. When the smoke clears, the onlookers are shocked to see the hollowed out torso of a titan with Mikasa and Armin safe inside.