"Onegai Teacher" is a romantic science fiction manga series written by Yosuke Kuroda and illustrated by Kōji Ōdate. The story centers on Kei Kusanagi, a high school student who appears to be an ordinary teenager but harbors a secret: he suffers from a rare condition called "Hibernation Syndrome," which causes him to fall into comas for extended periods, effectively halting his aging process. As a result, Kei is significantly older than his peers, though he appears to be their age. His life takes a dramatic turn when he encounters Mizuho Kazami, a mysterious and beautiful woman who becomes his homeroom teacher.

Mizuho is not an ordinary teacher; she is an alien sent to Earth on a mission to observe and study human behavior. Her presence is kept secret, and she lives in a spaceship disguised as a house near the school. When Kei accidentally discovers her true identity, Mizuho is forced to reveal her secret. To prevent Kei from exposing her, the two enter into a marriage of convenience, orchestrated by Mizuho's superiors. This arrangement complicates their lives as they navigate the challenges of maintaining their secret while dealing with their growing feelings for each other.

The narrative explores the dynamics of their relationship, blending elements of romance, comedy, and science fiction. Kei struggles with his feelings for Mizuho, torn between his affection for her and the societal taboo of a student-teacher relationship. Mizuho, on the other hand, grapples with her duty as an alien observer and her emotional attachment to Kei. Their interactions are further complicated by the presence of Kei's classmates, including his childhood friend Ichigo Morino, who harbors unrequited feelings for him, and Mizuho's alien colleague, Koishi Herikawa, who adds a layer of intrigue and humor to the story.

As the series progresses, the characters confront various challenges, including misunderstandings, external threats, and the complexities of their dual lives. The story delves into themes of love, identity, and the boundaries between duty and personal desire. The interplay between Kei and Mizuho's relationship and the broader sci-fi elements creates a unique narrative that balances emotional depth with lighthearted moments. The manga concludes with a resolution that ties together the central conflicts, offering a satisfying conclusion to the characters' journeys.