On the eve of the present battle, Bertholdt and Reiner recall the Battle of Trost when Marco Bott overheard them talking about Titans and their mission, simply by chance. They tell him they were just joking, but know Marco is sharper than that.
Annie soon joins them as they restrain him, and Reiner decides to remove his ODM gear. Shortly afterward he’s found and eaten by a Titan as the three watch in horror and remorse. Their secret was safe for a little longer, in exchange for their friend and comrade dying horribly.
Back in the present, while having coffee with War Chief Zeke (the Beast Titan) and discussing Annie’s capture, Zeke questions their commitment, but they resolve to end this with him tomorrow. At dawn, Bert and Reiner go to their separate positions, with the latter chiding the former for being so timid when he’s perhaps the most powerful Titan of them all.
Reiner encourages Bert to think for himself and not just wait for others to give him a signal to act. That’s all well and good if you’re the Armored Titan, but it’s a little different when your Titan is so big his transformation is basically a nuclear bomb.
Back to the latest point of the battle up to now, with half of Reiner’s head blown off, his Titan still manages to call out, which is the signal Bertholdt, hiding in a barrel, was waiting for. The Beast Titan tosses the barrel into the town. Irwin, Hange, Levi, Armin, and everyone else watch as it sails over them, and Armin realizes it’s Bertholdt is in there and they’ve got to get away.
But just when he seems ready to transform, which would be game over for everyone in the town, Bert spots the KO’d Reiner and hesitates. He fires up his ODM instead and rushes to Reiner’s side, where he learns Reiner transferred his consciousness to his nervous system as a last resort. Bert asks Reiner to flip himself over to protect his exposed nape when Bert transforms.
While Armin wasn’t able to negotiate with Reiner, he wants to at least try with Bertholdt, probably assuming the more timid of the two will be more willing to listen. But while Eren’s been mastering physical Titan hardening, Bert has been busy becoming mentally tougher, and meeting with Armin is the last bit of proof he needs to confirm that he’s finally ready to take the next step.
Bertholdt has no ill will towards Armin or his other former comrades; indeed, he regards them as “precious”. But they also have to die, because that’s what needs to happen. That’s the plan of all Titans, and those they serve. That it’s unfortunate doesn’t change the fact it’s inevitable truth that not even Bertholdt can avoid any longer.
Mikasa tries to take him out of the equation all together with a sneak attack, while Armin mentions Annie to try to throw him off emotionally, but Bertholdt shows he can and will deal with all psychological comers, and that he can live with whatever the result of this battle.
With that, Bert transforms, unleashing the nuclear-esque explosion virtually right on top of Hange Squad, while Levi and Armin’s squads just barely get to a safe distance. But no distance is safe once the transformed Colossal Titan starts scooping up flaming bits of town and launching them into the air, creating a kind of meteor shower for all surviving scouts to contend with.
Armin freezes up a bit, as the original plan to retreat back to Irwin and fight the Colossal in a battle of attrition will no longer work; not when Bert can put a wall of flame between Irwin’s forces and the Beast Titan. Armin’s only choice is to fight and defeat the Colossal Titan there and then, with only the forces he has. Needless to say, it will be a colossal undertaking, and they’re colossal underdogs.