The Soul Translator is a state-of-the-art full-dive interface which interacts with the user's Fluctlight—the technological equivalent of a human soul—and fundamentally differs from the orthodox method of sending signals to the brain. The private institute Rath aims to perfect their creation by enlisting the aid of Sword Art Online survivor Kazuto Kirigaya. He works there as a part-time employee to test the system's capabilities in the Underworld: the fantastical realm generated by the Soul Translator. As per the confidentiality contract, any memories created by the machine in the virtual world are wiped upon returning to the real world. Kazuto can only vaguely recall a single name, Alice, which provokes a sense of unease when mentioned in reality. When Kazuto escorts Asuna Yuuki home one evening, they chance upon a familiar foe. Kazuto is mortally wounded in the ensuing fight and loses consciousness. When he comes to, he discovers that he has made a full-dive into the Underworld with seemingly no way to escape. He sets off on a quest, seeking a way back to the physical world once again.

After a momentous New Year's vacation and with Valentine's Day approaching, Sawako Kuronuma is beginning to get along with her classmates. However, now that Sawako has realized her romantic feelings for the popular Shouta Kazehaya, she grows hesitant toward giving him obligatory chocolates and decides to not give him any. In turn, Kazehaya, who likes Sawako, feels a distance between them. As February ends and April arrives, the second year of high school begins for Sawako. Luckily, she ends up in the same class as her friends Ayane Yano and Chizuru Yoshida, along with Kazehaya and his friend Ryuu Sanada, in addition to the newcomer named Kento Miura. When Kento develops an interest in Sawako, Sawako and Kazehaya's feelings for each other are put to the test.

Koyomi Araragi is studying hard in preparation for his college entrance exams when he begins to notice something very strange: his reflection no longer appears in a mirror, a characteristic of a true vampire. Worried about the state of his body, he enlists the help of the human-like doll Yotsugi Ononoki and her master Yozuru Kagenui, an immortal oddity specialist. Quickly realizing what is wrong with him, Kagenui gives Araragi two choices: either abstain from using the vampiric abilities he received from Shinobu Oshino, or lose his humanity forever.

For high school student Tanaka, the act of being listless is a way of life. Known for his inattentiveness and ability to fall asleep anywhere, Tanaka prays that each day will be as uneventful as the last, seeking to preserve his lazy lifestyle however he can by avoiding situations that require him to exert himself. Along with his dependable friend Oota who helps him with tasks he is unable to accomplish, the lethargic teenager constantly deals with events that prevent him from experiencing the quiet and peaceful days he longs for.