The bad news? Rob Reiss transforms into an utterly massive Titan, far larger even than the Colossal. The good news? He’s no immediate threat to anyone in particular, just the structural integrity of the cavern and the chapel above it.
While neither of those two things survives the episode, everybody who set out to rescue Eren and Historia does. All the damage he causes allows Levi & Co. to escape Kenny’s trap and access the chamber where Eren and Historia are.
Even while free, Eren is lost. He was fully prepared to be eaten by Historia (and we later learn Historia momentarily considered transforming him and eating him so as not to be hated by her Dad). Between his antics and the sins of his father, Eren initially just sobs about being so worthless. His friends aren’t enthused. They risked their necks to get to him; the least he can do is not want to die, rendering their efforts in vain.
So Eren bucks up a bit, and bites down on a vial labeled “Armor.” The result of ingesting the serum within allows the Eren Titan to use the same Titan Hardening ability as the Armored Titan, building an avant-garde yet sturdy cage around his friends in order to save them from the growing Rob-Titan, which Levi dubs the “Big-Ass Titan.” It’s accurate!
Once they free Eren from his still fully-hardened Titan (sorry if that sounds a bit dirty), Levi & Co. take him and Historia and ge tthe hell out of there. As Armin and Moblit got the injured but stable Hange to safety earlier, the Scouts suffer no fatalities.
But there is that tricky matter of the Big-Ass Titan, which is a sight to behold; like something out of a Miyazaki film (Nausicaä, to be precise), yet like those Giant Warriors, not quite complete, and so it crawls awkwardly on its belly. Still, it’s so damn huge even in its current state it will reach the wall by dawn—and likely incinerate it.
When the initial plan to let Eren feed himself to Rob is shot down (there’s no guarantee Rob will return to being a human even if he eats Eren), the plan becomes “Stop the Big-Ass Titan at all costs.” Since Eren now has the Hardening Ability, there’s now a decent shot of repairing Wall Maria, but no repairs can take place until Rob is dealt with.
However, we do learn that Rob’s story about Eren’s father was naturally biased; in reality, Dr. Yaeger was trying to save humanity by destroying the line of the First King and the Reiss reliance on brainwashing. There’s also fresh mentions of the secrets in Eren’s basement, so there’s that.
Levi adds to Historia’s burdens by informing her that Erwin has insisted that once the latest battle is over, she will have to take up the mantle of Queen in order to give the regiment coup legitimacy and appease the public. Both Connie and Jean rightly pipe up that that shit ain’t fair, Historia has only just turned the page and gained as sense of individuality, only to be thrust into yet another duty.
I’ve really enjoyed Tori’s rise to prominence since her true identity was revealed; she’s basically a co-protagonist alongside Eren, and watching her decide her course in life in a world where such a thing’s a great luxury, has been a delight. She’s grateful for their defense, but she agrees without complaint…but with one request: If she’s to be queen, she’s going to give it all she’s got in the battle that proceeds her coronation.
That means donning the uniform of the Scout Regiment, strapping on her ODM gear, and taking her place beside her comrades—her friends—her true family—to whom she owes not only her life, but her sense of self. Rob isn’t her dad anymore; he’s the enemy. It’s time to go to work.