Shortly after their arrival, Mikasa, Connie and Armin join them in HQ just before a Titan smashes his face into the wall of HQ. As the cadets panic and move deeper into HQ, the good Titan attacks the bad ones and Connie tells Jean that they recruited a Titan.
While the good Titan fights the bad ones outside, the cadets find rifles, tucked away in a store room, and Armin comes up with a plan to take out the Titans downstairs, so they can get to the gas and get over the wall to safety. Armin’s plan involves blinding the Titans with the rifles, giving the most skilled at using the swords, time to take out the Titans.
After successfully completing Armin’s plan, most of the cadets flee over the wall, having refilled their gas, but Mikasa checks on the good Titan, finding him pinned to a house and being eaten by the other titans, at this point the Abnormal Titan we saw in an earlier episode walks past.
This has some sort of effect on the good Titan. It manages to get up, push past the bad titans and go after the Abnormal. The good Titan bites the Abnormal on the back of the neck and uses him as a weapon against the others, killing them before collapsing and vaporising - revealing something no one expected.
Starting with a flashback of a few hours before the Colossal Titan appeared, we get our first look at the Scout regiment, taking out a few Titans outside of wall Maria. We then join Eren in the stomach of the Titan he got swallowed by, a few episodes earlier.
Eren witnesses what it’s like inside the Titans stomach and vows to kill all the Titans while stretching half his left arm up into the air. Upon doing this, we see an arm shoot out of the mouth of the Titan that Eren is inside, followed by the rest of a Titan, which happens to be the good Titan that we witnessed killing Titans earlier.
We then find Eren being held by Armin with Mikasa, armed and ready to fight, protecting them both, surrounded by troops with their weapons drawn. The three of them are being shouted at and charged with treason while they try to find out what Eren is. Eren, confused to start with, slowly works out what happened and why he’s surrounded and being questioned about what he is.
After answering the question, the commander takes it upon himself to eliminate the “threat” and orders the cannon to fire on Eren, Armin and Mikasa. As the cannon ball hurtles towards them, Eren grabs Armin and Mikasa, shielding them and biting his hand causing a massive explosion. As the smoke clears, half a Titan sits with its arm stretched out into the air, where the cannon ball was coming from.
Eren wakes up inside and attached to, the neck of the half Titan at the start of episode 10 and pulls himself free. Mikasa and Armin are inside the Titan's ribcage, protected from the cannon fire and Armin notices that purple flowers have somehow grown - they weren’t there before. The half Titan begins to vaporise now that Eren is no longer attached to it. Eren joins Mikasa and Armin, checking to see if they're okay and remembers what his Dad said to him, when he gave him the key to the cellar.
Eren talks to Mikasa and Armin about what he plans to do when he escapes from the surrounding troops, who have just witnessed Eren turn into half A Titan. Eren tells Armin that he needs and trust him to get them safely out of the situation they are currently in.
Armin realises that Eren and Mikasa don’t see him as the small, feeble, useless boy he sees himself as compared to them, this gives Armin the courage and faith in himself to try and save himself and his friends. Dropping his fighting equipment, Armin runs out of the smoke and begins to try and persuade the surrounding troops that Eren isn’t dangerous, and that he is willing to work with the military and fight against the Titans, giving the Human race its first advantage over the Titan onslaught.
Before the leader of the troops can order another cannonball to be fired upon the three friends, commander Pixis shows up and decides to hear the three friends out. Armin tells Pixis of his plan to take back Trost, which started as a grabbing straw to save their lives, Pixis seems to think the plan could work and asks Eren if he’ll try and help take back Trost.
Eren agrees to try.