The Survey Corps organizes the 57th expedition beyond Wall Rose, with Eren serving as both bait and weapon to draw out the enemy. A mysterious Female Titan appears and demonstrates intelligence and selectivity, targeting the Survey Corps formation with brutal precision. Captain Levi's elite squad is annihilated in the Titan Forest, leaving their corpses impaled on trees as a message. Eren transforms to fight the Female Titan but is captured and carried away by her. Levi leads a desperate rescue mission and manages to extract Eren from the Female Titan's grasp. The Female Titan is eventually identified as Annie Leonhart, a member of the 104th Training Corps.
The Female Titan's ability to summon other Titans with her scream decimates the Survey Corps' long-range formation, killing scores of soldiers in a coordinated ambush. Levi squad members Gunther, Eld, Petra, and Oluo are killed one by one while attempting to protect Eren in the forest. Eren engages the Female Titan in combat and initially gains the upper hand through sheer rage and determination. The Female Titan hardens her skin to form armor and captures Eren by biting off his limbs, carrying him away from the battle. Levi intercepts the Female Titan using his Three-Dimensional Maneuver Gear at superhuman speed and rescues Eren, forcing her retreat. Commander Erwin's gambit to trap the Female Titan using the Survey Corps as decoys is partially successful but comes at an enormous cost in lives.
This arc introduces the reality that intelligent enemies exist among the Titans, fundamentally changing the nature of the threat humanity faces. The slaughter of Levi squad is one of the most emotionally devastating moments in the series, demonstrating that no character is safe. Annie's reveal as the Female Titan proves that the enemy has infiltrated humanity's ranks and has been training alongside the protagonists. The arc also showcases Erwin's ruthlessness as a commander willing to sacrifice soldiers for strategic advantage, establishing the moral complexity that defines the series.