Urado Shiori, a first year in high school, is saved by a mysterious monk named Ryudaiji. However, Ryudaiji is not your normal monk. He uses modern weapons combined with spells in order to exorcise demons that are terrorising the world. With his Gatling gun and two automated stone lions A-Gon and U-Gon, he saves Shiori from a vampire.
Urado Shiori is being held against a wall by a Vampire, who is planning on killing her. Shiori wonders if she is going to die and has a "lame" flashback. However, a Monk named Ryudaiji intervenes, who calls it a "sort of fate". The Vampire claims that he's always wanted to see a Monk (which he refers to the "Exorcists of Japan") for himself, and taunts Ryudaiji, asking him if he's going to start praying.
Ryudaiji says that his taunts are tiresome, and proceeds to attack the Vampire with a Gatling Gun, thus saving Shiori from death. After the Vampire claims he will have revenge, A-Gon and U-Gon appear, apologising to Ryudaiji for being late and asking him if he is okay. Ryudaiji says that he is fine, and informs them that they will return to base after he has cleaned up the mess. A-Gon and U-Gon wonder if they should protect Shiori, and Shiori passes out.
Shiori is woken up at school by her friends. Her friends wonder what she means by "Monk-san", "Gatling Gun" and "The Villain Passing By", and ask her what kind of dream she had. They then ask if she is ready for the test, much to her surprise. However, this is instantly forgotten when one of Shiori's friends gets a papercut on her finger. Her other friends offers her a band-aid, but doesn't manage to put it on before Shiori bites her fingers.
Shiori wakes up flying in the air with Ryudaiji, who asks if his fingers taste good to Shiori. She takes his fingers out of her mouth with embarrassment and apologises to Ryudaiji. She realises that she isn't in a dream, but doesn't consider it as real. Whilst stopping to buy something from a vending machine, Ryudaiji introduces himself to Shiori and states that she must be quite confused with what happened previously, calling her "finger-licking vomit girl". Shiori again feels embarrassed and thinks that her first impression is terrible. Despite her embarrassment, Shiori introduces herself, and tells Ryudaiji that she is a first year high school student. She then thanks Ryudaiji for saving her, but says that she will go back home. However, before is able to leave, Ryudaiji tells her that there's a doctor at where he stays and that she can go back home afterwards.
Shiori, Ryudaiji, A-Gon and U-Gon arrive at the temple. A-Gon and U-Gon say goodnight to Ryudaiji and leave his side. Shiori gets distracted by a shrine, but is then beckoned by Ryudaiji to follow him into the temple. After entering, Ryudaiji warns Shiori to maintain her footing for the descent. To Shiori's surprise, the lift drops quickly. Once they reach the bottom, Shiori is amazed by the underground headquarter that has been set up underneath the temple, but wonders why it is where it is.
Shiori is then greeted by Yaobikuni, who is scolded by Ryudaiji. Ryudaiji then confirms that Yaobikuni is his great (x9) grandmother, which surprises Shiori. Yaobikuni informs Shiori that she was taken to have some experiments on her body a long time ago, which made her immortal, to which Shiori chides her for not telling her sooner. Yaobikuni proceeds to claim she is quite pleased that Ryudaiji has brought "his girlfriend" home with him, to which Shiori denies.
Yaobikuni informs Shiori of the Hazama (the demons of the inbetween) while the latter is having blood tests taken. Ryudaiji interjects, saying that what attacked Shiori is referred to as a "vampire". Shiori replies that they sound like foreign monsters. Shiori is then told that Hazama have integrated themselves into human society and that Japanese are often targets due to the fact that they have a "low guard". Yaobikuni recalls how she ran into a Vampire 100 years prior and how their rate of regeneration was much worse. She then enthusiastically claims that Monks should evolve along with the vampires.
After Shiori's blood is taken, she apologises to Ryudaiji for everybody working hard to help her eventhough they had just met her and proclaims that she's useless. However, Ryudaiji remarks that it doesn't matter if she is useless or not and that it is a Monk's duty to help anybody who needs it.
Later on, Yaobikuni hands Shiori a photo of "Ta-Kun" in middle school. Shiori is surprised by the photo, to which Yaobikuni responds with "Awesome, right?". She then mentions about how his personality and environment made him into "such a gloomy person". After this, Yaobikuni falls asleep, and Shiori claims she suddenly feels tired. She proceeds to check her phone and seems shocked.
Ryudaiji talks with the doctor that did Shiori's blood tests about her results. The doctor mentions that some of the cells in her body have mutated to alarming degree and that that should prepare something, but he is interrupted by Yaobikuni, who is surprised that Shiori wasn't present. Ryudaiji then vows to "end it" before Shiori becomes a vampire.
Shiori is seen walking on her own, albeit being watched. She thinks that she may not turn into a vampire after all and that she should head back, but her thoughts are interrupted by a machine. It then attempts to eliminate Shiori (without any success). She manages to get away from the machine, but is then spotted by the same vampire from earlier. Shiori starts to transform into a vampire, to which the vampire is pleased about. The vampire attempts to feed Shiori a bottle full of blood, but Ryudaiji intervenes before he is able to.
The vampire uses Shiori as a human shield, so that she would get killed as well if Ryudaiji attempted to shoot the vampire. Shiori tells Ryudaiji to kill her as well so that she doesn't fully turn into a vampire, but Ryudaiji refuses. He and the vampire then proceed to fight, but eventually Ryudaiji wins. Along with this, Ryudaiji manages to get rid of the vampire blood in Shiori's body.
Ryudaiji is then seen in a flashback talking to a middle school student after saving him, showing a similar situation to the current one. After Ryudaiji finishes talking about fate, Shiori enthusiastically thanks him for saving her. After the vampire gets back up, Ryudaiji proceeds to attack him, while Yaobikuni and the doctor watch on a camera.
The next day, Shiori's friends mention how low her test result is, to which Shiori tells them both that she had busy day the day before. Her friends ask her if she wanted find a ghost that was rumoured to hang around a spotlight near the school, but Shiori refuses. She then says that a monk will send it to the afterlife if the rumour is indeed true.