The Assault on Stohess (ストヘス区急襲 Sutohesu-ku kyūshū?) was a conflict between the Scout Regiment and the Female Titan within Stohess District, the eastern city of Wall Sina, in which the Female Titan was captured and the existence of Titans within the Wall was discovered.
During the 57th Exterior Scouting Mission, Armin Arlelt encountered Annie Leonhart, the Female Titan, and was able to deduce her identity after considering her actions during the expedition and seeing her Titan's face up close. After the Scout Regiment's return to Calaneth District, he shared this deduction with Commander Erwin Smith, who devised a plan to capture Annie in Stohess District, Annie's base of operations in the Military Police Brigade and the half-way point between Calaneth and the capital where the Scout Regiment had been summoned to hand Eren Jaeger over to the MPs. This plan was shared with Eren, Mikasa, and Captain Levi first, though Eren was in great disbelief and denial at the possibility that Annie was the Female Titan.
While a majority of the Scout Regiment made their way from Calaneth to Stohess on their way to the capital, a squad headed by Miche Zacharius supervised a group of new Scout Regiment recruits, leading them to an isolated base in Wall Rose where they would be under close watch. This was done to isolate Annie from any potential accomplices who could aid her during the Stohess District raid.
Annie Leonhart, having returned to Stohess District after her failure to capture Eren Yeager, began to oversleep in the days to come due to the great stress brought on by her prolonged time spent in multiple Titan forms.
On the morning of the raid, the new recruits of the Military Police Brigade, including Annie Leonhart, were assigned with escorting the Scout Regiment through Stohess District on their way to the capital. As Annie joined her comrades on their way to the escort mission, she was beckoned aside by Armin who informed her of a plan by the Scout Regiment to sneak Eren out of the district. After some convincing, Annie agreed to help Armin and the others. Suspecting a trap, Annie equipped herself with a blade-ring in order to trigger a Titan transformation if necessary.
With Eren and Mikasa, the four made their way to the entrance of a tunnel of an abandoned underground city which would lead them to the outer gate of Stohess. In reality, the tunnel was chosen to restrict Annie's ability to transform into a Titan and Scouts would await them below to capture her. Meanwhile, various members of the Scout Regiment in civilian disguises waited out of sight in the event that the first plan failed. Annie hesitated to follow them underground, and the others questioned her choice with as little suspicion as possible.
However, their conversation quickly became heated as Annie realized she was cornered, no longer denying her identity as a Titan. Mikasa became impatient and prepared to strike as Annie readied herself for Titan transformation, but Armin fired a signal flare to alert the disguised soldiers of the plan's failure. The soldiers quickly rushed Annie and restrained her, but Annie successfully slashed her thumb with her blade-ring and transformed, killing the soldiers around her as Eren, Mikasa, and Armin fled into the tunnel.
The three recruits attempted to meet up with the soldiers in the tunnel to initiate their plan B, but Annie's Female Titan stomped a hole into the tunnel, killing the team before any action could be taken. Eren proposed transforming into a Titan in order to shield himself and his friends from Annie, but his internal denial about Annie's betrayal prevented him from transforming. Armin proposed that he and Mikasa would split up and leave the tunnel from both the entrance and the hole left by Annie's foot at the same time, dividing her attention in order for Eren to safely escape. Eren wondered at his friends' ability to accept Annie's betrayal, and Mikasa simply attributed it to the cruelty of the world. Before Eren can take action, he is crushed beneath the rubble of the collapsing tunnel as the Scouts above battle the Female Titan. Jean and Armin manage to lead Annie into a trap set by Section Commander Hange with special target restraining weapons, but Annie manages to break free. Armin meets with Eren and reminds him of the necessity to abandon humanity to overcome monsters. Eren accepted this and cruelty of the world found the strength to transform, engaging in battle once again with the Female Titan.
In his Titan form, Eren engaged in combat with Annie in the streets of Stohess District. Upon hearing the Titans' brawl elsewhere in the city, the Scout Regiment's escort came to a halt as Commander Nile Dawk sent his recruits to investigate. Commander Erwin Smith advised him to dispatch all available Military Police officers, and the escort to the capital ceased.
The fight between Annie and Eren continued as the two caused significant damage to the structures of Stohess, including a church where Pastor Nick was giving a sermon to his followers of the Order of the Walls. Annie eventually disengaged and attempted to reach Wall Sina and escape, but her brawl with Eren continued for a short while before Eren was briefly incapacitated. In that moment, Eren's Titan entered a feral state and rushed the Female Titan as she fled for Wall Sina, tackling her and ripping a leg from her body as she had begun to scale Wall Sina using her hardening ability to dig her fingers into the stone. Before Annie could reach the top of the Wall, Mikasa maneuvered up to Annie's position where she sliced the Female Titan's fingers. Annie fell to the ground below and was quickly pinned to the ground by Eren's Titan, who immediately began to tear at the flesh of her nape while ripping her Titan body to pieces. Once she was exposed in her Titan's nape, Annie made a last-resort move and encased her body in impenetrable crystal with her hardening powers, preventing her interrogation while resigning herself to capture.
By the end of the battle, over a hundred people have been killed in the fight between Eren and Annie. The Scout Regiment made quick efforts to relocate Annie's crystal underground to prevent future Titan transformations. At the same time, the Scouts present spotted a gap in Wall Sina where the Female Titan had damaged the stone, revealing the face of a Colossal Titan within. Pastor Nick quickly arrived at the scene and urged Section Commander Hange Zoë to cover the Wall Titan to prevent the sunlight from touching it. Hange's team gathered necessary supplies to cover the hole. Later that afternoon, leaders of the Scout Regiment were summoned to the Military Police Brigade Stohess branch headquarters to answer for the events of that day. Erwin Smith stated that information concerning the mission was kept hidden from the Military Police due to the possibility of accomplices of Annie Leonhart acting within the brigade. Annie was confirmed to be detailed underground, out of commission despite the impossibility of interrogating her for information. Shortly afterward, Erwin was informed of the Wall Titan spotted in Wall Sina.
Once the Wall Titan had been successfully covered, Hange interrogated Pastor Nick concerning his knowledge of the Wall Titan. Despite threats against his life, Nick refused to reveal any information. Soon afterward, Scout Regiment soldier Tomas arrived at Stohess to warn the Scout Regiment leaders of a Wall Rose invasion that had been occurring at the time of the Stohess District mission. The members of the Scout Regiment present in Stohess District immediately assembled and made their way southwest to Ehrmich District to assist in the evacuation of Wall Rose.