Detectives These Days Are Crazy!

Private Detective Keiichiro Nagumo tries to track down a suspected cheating wife when his back flares in pain. Before he can make it out the door, a hyperactive high school girl shows up unannounced and requests to be his assistant. He refuses and follows his target to a nearby café. When the same girl shows up again, she helps him complete the job by pretending to be his "age gap" girlfriend despite his loud protestations. After getting photographic proof of the cheating wife for the client, Nagumo reluctantly lets the girl stay on as an unpaid apprentice, and she reveals her name as Mashiro.

After taking on a stray cat as their unofficial mascot, Nagumo tasks Mashiro with another client's request to exterminate a bees' nest. However, a lack of clarity on the address leads Mashiro to the office of a yakuza group, but she proceeds to destroy it anyway. In the process, she scares a low-level member named Nezu to quit before she kills him, and Nezu repays her by joining Nagumo's office.

Mashiro's classmate Hayashida visits the office with a request to investigate a spooky ghost that appears near his home. Mashiro has phasmophobia, but Nagumo convinces her to investigate with him anyway. After a week, Nagumo realizes that Hayashida has a crush on Mashiro, and recorded himself disguised as a ghost to replay the clip each time Mashiro came over because he wanted an excuse to talk to her. Following Hayashida's confession, Mashiro returns with a real captured ghost.

Nagumo is held by the Tokyo MPD under suspicion of being an underwear thief, where his former classmate, Soya Mimasaka, is the one interrogating him. However, the person caught on camera stealing underwear looks a lot like Nagumo, so Mimasaka refuses to release him until Mashiro and Nezu set a trap to catch the real thief and bring him to the station.

A captured suspect called "Knit Man" reveals he planted a bomb in a nearby parking garage. Mashiro attempts to disarm the bomb, but runs low on time after taking multiple selfies next to it. In the end, she opts to kick it inside an abandoned baseball park. Unbeknownst to Mashiro, Nezu is there taking pictures when the bomb explodes.

Nagumo laments the fact that he has no money to pay for his office's rent when Yuu Asunaro, himself a young private detective inspired by Nagumo's former exploits, breaks into his office and offers to be his unpaid apprentice. However, Mashiro immediately gets jealous and challenges him to a duel. Asunaro's own assistant, Hana "Maki-chan" Kazamaki, acts as referee. While the two rush off to complete a tiebreaker, Nagumo tells Maki that he has no intention of firing Mashiro and that he happy that he serves as Asunaro's inspiration. Mashiro then manages to win the race by unleashing numerous objects buried somewhere within her sailor uniform and making herself lighter.

Asunaro shows up at Nagumo's office again, but this time with a case: to find out whoever is stalking him. After Asunaro goes through clumps of his adoring female fans, Nagumo spots a man in a hoodie stalking him from farther out. After Maki manages to restrain him, the group uncovers that Nezu is the stalker. However, he claims that he was actually spying on the "real" stalker, Asunaro. When Asunaro confesses to watching Nagumo's office over the past month, he escapes from Maki's wrath.

Nagumo and Mashiro are invited to a dinner party at a place called the "Mansion of Screams" where the wealthy Nagasaki family was supposedly massacred 15 years ago. As the guests gather inside, Mashiro and Asunaro both assume a mystery is about to unfold. When Mashiro heads outside to check if the rope bridge to the mansion is burning, she accidentally destroys it. Not wanting to admit this, Nagumo and Mashiro play along when another guest suggests there is a killer at the party trying to recreate the family massacre. Asunaro gets dragged into the act and accuses one of the guests, Megumi Shiraishi, of being the killer. To everyone's surprise, she admits to planning her revenge after being the sole survivor of the massacre. Back at the office, Mashiro tells Nagumo that Shiraishi's family was actually alive and Kuroi was at the dinner the whole time, so everyone was misguided at the party.

Maki joins Nagumo at a bar and both rant about how modern culture has passed them by. Maki soon realizes she has been too honest about her age and awkwardly tries to take back what she said.

Nagumo takes Mashiro to a fancy bar called Tonto. There, the barkeep eventually tells Nagumo that someone has been repeatedly spraying graffiti outside. Nagumo quickly deduces that Veronica, one of the regulars, is the culprit. Afterwards, Mashiro swears never to go back to that bar.

An anonymous client sends a request to the Nagumo Detective Agency with an advance payment. As Asunaro and both assistants join them, they suddenly get locked into a building where "Mister X" challenges them to a quiz game. Despite failing most of the questions, Maki and Mashiro combine their talents to uncover the room where Mister X is hiding. Maki then beats him up.

Mashiro shows off her latest purchase, an expensive wiretap detector, and finds that someone planted a bug on the floor below. After a while, Nagumo introduces Mashiro to the self-proclaimed "star inventor of Shinjuku," Azuha Hoshino. Azusa uses her latest invention, a megaphone, to overload the audio bug, and the group finds a pizza deliveryman outside clutching his ears in pain as the culprit. The following day, Mashiro uses Azuha's megaphone to wake up Nagumo, but also shakes out multiple thieves who were hiding in the office.

Mashiro hosts a Girls' Night Out with Maki and Azuha at a nearby izakaya. While a drunk Maki refuses to reveal her real age, she does admit that she admires Asunaro and the reason why she that she likes to wear a poncho. Elsewhere, Nagumo struggles with the menu at a different izakaya while Nezu and Asunaro argue with each other.

Nagumo gets sick and tired of Mashiro breaking his property, resulting in her storming out and switching places with Maki for a day. Maki helps Nagumo with interviewing the case of someone being stalked and harassed in their neighborhood. Meanwhile, Asunaro and Mashiro are looking for a pervert that has been terrorizing the community. After said pervert appears, Mashiro struggles to catch him, but chases him down to the same neighborhood where Nagumo and Maki are. Afterward, Nagumo admits that he was bored without Mashiro around and welcomes her back.

Nagumo tries to enjoy a sauna when an escaped criminal suddenly enters. The two try to one-up each other until an old man fills the sauna with so much steam it knocks both men out. Nagumo wakes up in the locker room to find the suspect was re-arrested.

Mashiro wants to drink black coffee to seem like a "mature" woman to Nagumo. Azuha offers her a pill to help her, but Mashiro soon finds out it comes with aphrodisiac-like side effects. After Mashiro apologizes, Nagumo offers her a café au lait instead, though she spits it out anyway as the milk used for it was sour.

A lost child calling herself "Beaver Fu" asks Mashiro for help finding her parents. Nagumo eventually deduces from Fu-chan's actions that she already knew Mashiro and Asunaro before meeting them. Suddenly, Mimasaka appears and addresses Fu-chan as his daughter. Fu-chan then proclaims that she will become the "Phantom Thief Beaver" and steals Nagumo's wallet.

Fu-chan demonstrates her love of thievery by stealing various items from Nagumo's office when the local news reports that a Phantom Thief named Riverfake has left a calling card that he intends to steal "The Smile" painting. While Fu-chan is enraptured by Riverfake, her father calls in Nagumo and Mashiro to help out. Riverfake is ultimately taken down by Fu-chan. Afterwards, Fu-chan demonstrates one last act of thievery by stealing Nagumo's present from the gift box that Mimasaka sent to him.

After interrogating a man who claims to have no memory of the crime he committed, Maki believes a VTuber named Yomoya Hikari might be using their YouTube channel to spread "digital drugs" that brainwash their followers into committing crimes without realizing it. The group watches one of the VTuber's videos to test the claim, only for Mashiro to suddenly turn into a monster and jump out the window. Maki tries to get the cops to help but they only arrest a local pervert instead of Mashiro. As Mashiro struggles to fight the brainwashing, Maki tries commanding her to go home like a dog, causing Mashiro to go to the "home" of the men behind Yomoya Hikari instead and chase them into the open where the "Big Four Creepy Old Dudes" take them down.

Fu-chan spends the day practicing her Phantom Thief skills, only to be foiled by the eccentricities of the Big Four Creepy Old Dudes.

Maki goes through a routine on her day off when Asunaro calls her in the evening to work on another case.

When Mashiro loses her smartphone, she discovers it is in the hands of an unknown thief. The thief tells Mashiro to complete a series of seemingly impossible tasks for hints on where he left the phone. To his surprise, she completes every task given. Elsewhere, Asunaro and Maki talks about the "Serial Smartphone Molester", who tries to make women lose his impossible challenges so he can enjoy their agony and shame up close. As the man decides to give up on Mashiro, Fu-chan steals Mashiro's phone back, and tortures him with the help of Nipple-Tasting Man, before turning his true identity over to her father. It is later revealed the Serial Smartphone Molester tried the same act on Azuha the previous day, only for the phone to literally blow up in his face.

At Nagumo's birthday party, Asunaro recounts how Maki became his assistant. Nagumo then recounts the time he opened his detective agency. This is followed by Asunaro recounting his first case with Maki.