"Mysterious Girlfriend X" centers on the peculiar relationship between Tsubaki Akira, a high school student, and Mikoto Urabe, a transfer student with an enigmatic demeanor. Urabe is known for her aloof personality, unusual habits, and a mysterious drool that she secretes, which Tsubaki accidentally ingests early in the series. This incident creates an inexplicable emotional and psychological bond between the two, leading Tsubaki to develop an intense attachment to Urabe. Despite her stoic and often cryptic behavior, Urabe reciprocates his feelings, and the two begin a tentative romantic relationship.

The narrative explores their evolving connection, which is marked by Urabe’s unconventional quirks, such as carrying scissors in her underwear and her fascination with Tsubaki’s drool-induced dependency. The series delves into the dynamics of their relationship, emphasizing the challenges of communication and intimacy as they navigate their feelings for each other. Tsubaki’s curiosity about Urabe’s past and her mysterious nature drives much of the plot, though Urabe remains guarded, revealing only fragments of her background.

Supporting characters, such as Oka Ayuko and Ueno Kouichi, provide additional perspectives on relationships and adolescence, often contrasting with the central pairing. Ayuko, Tsubaki’s childhood friend, harbors unspoken feelings for him, while Kouichi serves as a confidant and occasional comic relief. The series balances its exploration of romance with moments of humor and surrealism, often highlighting the awkwardness and uncertainty of teenage emotions.

The story progresses through episodic encounters that deepen the bond between Tsubaki and Urabe, while also addressing themes of trust, dependency, and the boundaries of love. Urabe’s mysterious drool remains a recurring motif, symbolizing the intangible yet profound connection between the two characters. The series concludes with an open-ended resolution, leaving their future together ambiguous but emphasizing the growth and understanding they have achieved.