
Chapter 31 of Attack on Titan springs the trap at the heart of The Female Titan arc: Armin lures Annie Leonhart into an underground escape route in Stohess, confronts her with everything he has pieced together, and watches her mask fall away with a chilling smile.
Annie oversleeps in Stohess District while the townspeople trade rumors about Eren's transfer to the Military Police that day, with a Wall-cult fanatic railing against anyone who would dare seal the breach. When her unit assembles, a senior officer dumps responsibility for escorting the Survey Corps convoy onto Marlowe, the recruit who dared to ask a question. Outside, Marlowe rants about the rot inside the Military Police and his ambition to reform it from within. Annie tells him his defiance reminds her of someone, then adds that most people, herself included, are the self-serving "scum" he despises; that is simply what normal looks like.
From an alley, Armin quietly asks Annie to help smuggle Eren out of the city. She calls the plan half-baked and refuses, relenting only when Armin insists Eren will die otherwise and turns her own question back on her: if she walks away, she becomes a bad person in his eyes. Unnoticed, she slips a silver ring onto her finger. Disguised in hooded raincoats, Eren, Mikasa, and Armin move through streets the inattentive Military Police barely watch, with Jean riding in the convoy as Eren's decoy.
At the mouth of a tunnel to the Underground, Annie stops. Her excuse, that she is too frail and frightened a maiden to go below, convinces no one, and she notes aloud that the streets around them have been strangely empty. Armin then lays out his case: the maneuvering gear she carried was Marco's, and she is the one who destroyed the captive Titans Sonny and Bean. He admits he stayed silent because he wanted his reasoning to be wrong, and asks why the Female Titan spared him in the field. Eren offers her one last chance to come quietly, but Mikasa throws off her coat and draws her blades. Cornered, Annie breaks into a wide grin, raises her hand to her mouth, and declares that their gamble may have paid off, but hers is only beginning. Hidden Survey Corps soldiers watch from the rooftops.
Annie joins the Military Police detail escorting the Survey Corps through Stohess, and Marlowe is saddled with command of the operation. Armin recruits Annie into a supposed plot to help Eren escape, the bait in the Corps' real plan to capture her inside the city.
Annie's refusal to enter the tunnel confirms Armin's suspicions. He openly accuses her of using Marco's maneuvering gear, killing the research Titans Sonny and Bean, and being the Female Titan.
With Mikasa ready to cut her down and soldiers concealed throughout the district, Annie abandons the pretense, grinning as she moves to bite her finger and transform.
This is the final chapter to carry a Current Publicly Available Information insert; entry fourteen details the Military Police Brigade, its roughly 2,000 members, its command over the Garrison, and its friction with the Survey Corps.
Cornered at the tunnel, Annie says in the Japanese text that she failed to become a "Warrior" (Senshi), a term Kodansha's English edition rendered as "soldier", understandable since the manga did not distinguish Warriors from soldiers until Chapter 39. The chapter opens volume 8 and is adapted in the anime as "Smile: Assault on Stohess, Part 1", following "Losers" and preceding "Mercy".

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Chapter 31, "Grin," springs the trap of the Female Titan arc: Armin lures Annie Leonhart into an underground escape route in Stohess and confronts her with everything he has pieced together, prompting her mask to fall away with a chilling smile.
Armin notes that Annie carried the maneuvering gear belonging to the dead soldier Marco and that she destroyed the captive research Titans Sonny and Bean. Her refusal to enter the escape tunnel confirms his suspicions.
Cornered at the tunnel with Mikasa ready to attack and soldiers hidden throughout the district, Annie abandons her pretense, breaks into a wide grin, and raises her hand to her mouth to transform.
The "Grin" chapter is adapted as the anime episode "Smile: Assault on Stohess, Part 1," following "Losers" and preceding "Mercy."
"Grin" is the final Attack on Titan chapter to carry a Current Publicly Available Information insert, detailing the Military Police Brigade's roughly 2,000 members, its command over the Garrison, and its friction with the Survey Corps.
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