Mitsugu Bamba, who is crazy about donating her blood, meets a beautiful girl who destroys the local blood bank that she frequents. The girl turns out to be a vampire named Mai Transilvania, who has run away from home. Mai, who doesn’t like forcing humans to bend to her will and is unable to attack humans, had been wandering around hungry and homeless. Mitsugu decides to let Mai live with her, and devises a plan to gather food (blood) for Mai.

With the help of Chihiro, the school nurse without morals who studies blood, Mitsugu starts a Blood Donation Club at school to collect food (blood) for Mai. She successfully appeals to the students’ desires and attracts some unique potential members to the nurse’s office. That night, Mitsugu gives the blood she collected to Mai, and realizes her personality changes depending on the blood she drinks. Then, Mai drinks the forbidden blood that was stored in Mitsugu’s house and…

Mitsugu learns that Mai is not afraid of crosses or garlic. Worried that Mai’s traits don’t fit her image of vampires, Mitsugu decides to further investigate. Eyeing Mai’s cosplay, Kaoru, captain of the Cosplay Club, invites them to a Halloween cosplay party. Not knowing Mai’s real identity, Kaoru dresses her in a vampire cosplay!?

Mai leaves quite the impression at the Halloween party and becomes the center of attention amongst the students at Seijumonji High School. Wanting to know the secret behind Mai’s odd behavior, the students pester Mitsugu, Mai’s de facto guardian. Mai and Mitsugu escape to the Chimatsuri Research Institute, but chaos ensues as the students flood the facility. Undisturbed by the chaos, Mai drinks the forbidden blood again and…

Mai’s true identity as a vampire becomes an open secret. To continue protecting Mai, Chihiro establishes a night school at Seijumonji High School so Mai, Mitsugu, and others can enroll. Meanwhile, Mai’s dad, Gilles de Rais, suddenly visits Mitsugu’s home, and Mai is excited to see him. Meanwhile, Mitsugu’s dad returns from abroad, and Mitsugu learns that her dad and Gilles de Rais have been rivals for decades.

The night school at Seijumonji High School finally opens. As its first students, Mitsugu and the others decide to put on the play Castlevania for the school festival. However, stubborn, perfectionist, and Type A Jinko of the Discipline Committee takes over as the director, and Mai becomes stressed. On the day of the festival, Mai faints from anemia. Chihiro decides to give Mai Jinko’s blood, but…!?

Maki, captain of the Cinema Club, decides to make a blood donation promotional video, and creates a super busy production schedule. Since the ideas and script for the video only exist in Director Maki’s head, the filming doesn’t go quite as planned. Maki forcefully wraps up the video by yelling at the students via a megaphone and convincing them with geeky film talks.

Inside a blood vial labeled “Memories” is an ancient, mysterious blood. What is this blood that Chihiro, teacher and advisor of the Blood Donation Club, treasures? The students are itching to know, and Chihiro starts telling its story. The story is about a young girl and a boy who experience a dramatic adolescence together. The long story that inspires Chihiro Chimatsuri, the incomparable blood researcher, is unveiled.

Blood continued to drip from Mitsugu’s left arm. Mitsugu, who is at the beach, has been bitten by a jellyfish. Mitsugu looks for a doctor to stop the bleeding, wanting to save her precious blood for Mai. Possibly hallucinating from losing so much blood, Mitsugu starts seeing the world around her and her friends differently. Is this a dream or reality? The doctor who she finally finds is of course that person…

Yasohachi Franken is the new student at the Seijumonji High School’s night school. Turns out this artificial human who stops moving when he’s not plugged in is Mai’s fiancé! Although Yasohachi is intelligent and athletic, Mai insists that their engagement was arranged by her parents and is cold towards him. One day, he is hit by lightning, and frantically searches the streets for Mai.

Drawn to a cheap, 10,000 JPY per person 5-night 6-day vacation package, the Blood Donation Club members go to Unbaba Island, an island not listed on any map, for a retreat. They end up spending four days going to/from the island, and the inn where they’re staying is falling apart, but the students try to enjoy their stay. Then, Nami, captain of the Dance Club suddenly disappears, leaving behind her swimsuit. When the students go looking for her, the old lady at the inn tells them a terrifying story about an incident that happened on the island.

Makin, captain of the Cinema Club, approaches Mitsugu about creating a documentary about Mai. Mitsugu initially refuses, saying she doesn’t want their peaceful life disrupted, but eventually gives in. Becoming impatient with Mai, who refuses to talk about her past during filming, Chihiro mixes a sleeping pill with Mai’s food (blood), forcing her to be interviewed in her sleep. Mitsugu learns about Mai’s sad past, but!?