Atsushi Nakajima and Jun'ichirō Tanizaki vs. Lucy Maud Montgomery

Atsushi Nakajima and Jun'ichirō Tanizaki vs. Lucy Maud Montgomery is a battle fought between Armed Detective Agency members Atsushi Nakajima and Jun'ichirō Tanizaki against Lucy Maud Montgomery, an apprentice from the Guild during the arrival of the organization in Yokohama in pursuit of a certain Book.

It happened in Lucy's dimension known as Anne's Chamber after the participants were transported from the streets of Yokohama.

Prologue
With Kenji Miyazawa's disappearance following the visit of Francis Scott Key Fitzgerald to the Agency office, Doppo Kunikida tasks Atsushi and Tanizaki to find Kenji, while Naomi Tanizaki volunteers to accompany them. Along the busy streets of Yokohama, Naomi suddenly vanishes in broad daylight, similar to how a man in white claims that his companion, Elise, has also disappeared. Eventually, Tanizaki spots Lucy among the crowd and chases her, only for him and the civilians in close proximity to be transported into her dimension.

Lucy welcomes her guests to Anne's Chamber, upon which Tanizaki impatiently asks where Naomi is. Lucy shows the Agency members their captured subordinates in her secret room, challenging them to play with Anne, a giant doll-like creature, if they wish to free their comrades from imprisonment. Out of fright, the civilians flee out the door back to the streets, an act which Lucy ascertains will lose their memories in the Chamber.

As Atsushi, Tanizaki, and the man in white – who refuses to leave Elise behind – face Lucy, she explains the rules of their game with Anne, in which getting caught by Anne results to their loss. Moreover, as she shows a key to the secret room, Lucy asserts that they win if they can open the door before getting caught. Lastly, Lucy prohibits the use of violence that might damage anything inside the Chamber. Atsushi and Tanizaki accept the rules and decide to participate in the game.

Battle
Lucy launches the game, upon which Anne almost instantly catches Tanizaki. He is dragged to the secret room and gets imprisoned, while Atsushi quickly evades Anne's succeeding attempt to catch him using his tiger limbs, much to Lucy's surprise. Anne's hands snake out in a flurry, but Atsushi's speed and reflexes allow him to evade Anne.

Lucy is amazed with his acrobat skills and ability, surmising that he must have been doted upon since his childhood. She reveals being an orphan like him, regarded as a freak most especially due to her ability. She then spites him for being the one given attention to, citing that she heard of the Agency saving him when he was abducted and that the Guild awaits no failure from its operatives. Filled with bitterness, Lucy asks him to stay in the Chamber forever, and Anne's hands immediately assault him once again.

Atsushi dodges, throws some gift boxes at Anne's direction as diversion, and rushes straight to retrieve the key. As he successfully grabs it and tries to unlock the door, the man in white yells at him to look out, as the key writhes semi-sentiently and morphs into a blade that lashes out towards Atsushi.

He quickly tosses out the object, while Lucy reprimands him for carelessly throwing it out and reveals having no idea herself on how to use the key. Appalled by how they do not seem to have a chance to win right from the start after all, Atsushi is caught off-guard when Anne descends upon him once more. Atsushi continues to flee using his tiger speed and tries to conjure an idea, but he gets entangled in some ribbons trying to evade capture.

With no more options left, Atsushi heads to the exit door to seek help from Osamu Dazai and the others. However, the man in white pulls him in the ribbons he is entangled in, preventing him to leave the Chamber. Instead, the man offers him game theory advice to beat down their opponent on the spot and reminds him that his subordinates trapped in the Chamber are the same ones who saved him before from what he heard.

Atsushi then opts to retaliate per the man's strategy, but a second Anne emerges from the floor, which causes Atsushi to leap above and get caught by the first Anne. He then gets dragged into the secret room, passing by Lucy in the process. Lucy then turns to the man in white and thinks of what to do with him. To her horror, she senses the man's killing intent that even renders Anne frozen in fear. He then declares her defeated, pointing to Atsushi resisting from getting dragged using his tiger limbs courtesy of Tanizaki's Light Snow, which makes it appear that Atsushi has been truly dragged in.

Atsushi soon uncovers a ribbon tied to her waist, which he reveals he has tied to her waist when he was getting dragged by the Chamber's hands. He then pulls Lucy towards him and demands her to deactivate her ability at once, otherwise threatening to let go and drag her with him to imprisonment in her own dimension. He remarks that abilities must not be used to dominate over others, despite Lucy's desperation not to fail her mission. Atsushi then gives her a split second to decide on their fate and lets go. At that moment, they are all returned to the streets, with Kenji, Naomi, Tanizaki, and the rest of the imprisoned released from the secret room.

Aftermath
Atsushi spots Lucy nearby and apologizes to her, but Lucy turns a frustrated gaze at him and runs away weeping. The man in white finally gets reunited with Elise, while Kyōka Izumi runs towards Atsushi and embraces him out of relief.

The man imparts a final advice to Atsushi about not giving up in every ordeal before departing with Elise. Meanwhile, Kyōka falls to her knees, visibly horrified upon seeing the man in white who turns out to be the boss of the Port Mafia himself.