"Tenchi Muyo! The Night Before The Carnival" is a 1999 OVA set in the expansive Tenchi Muyo! universe, serving as a prelude to the events of "Tenchi Muyo! GXP." The story unfolds as Tenchi Masaki and the eclectic group of women living with him prepare for a local carnival. The narrative centers on the arrival of Seina Yamada, a young man notorious for his extreme bad luck, who becomes entangled in the chaotic lives of Tenchi and his companions. Seina's presence inadvertently triggers a series of comedic and dramatic events, including misunderstandings, romantic tensions, and supernatural occurrences, all while the group attempts to organize their carnival activities.

The main characters include Tenchi Masaki, a kind-hearted but often overwhelmed young man; Ryoko, a fiery and mischievous space pirate; Ayeka, a dignified and sometimes haughty princess from the planet Jurai; Sasami, Ayeka's younger sister with a mysterious connection to the goddess Tsunami; Mihoshi, a clumsy yet well-meaning Galaxy Police officer; and Washu, a brilliant but eccentric scientist. Seina Yamada, the catalyst for much of the story's conflict, is portrayed as an unlucky yet resilient individual whose misfortunes often lead to unexpected outcomes.

The plot intertwines slice-of-life moments with science fiction elements, as the characters navigate their relationships and responsibilities while preparing for the carnival. The OVA explores themes of camaraderie, fate, and the unpredictability of life, all while maintaining the lighthearted and humorous tone characteristic of the Tenchi Muyo! franchise. The story concludes with the group successfully hosting the carnival, though not without a fair share of chaos and heartfelt moments, setting the stage for future developments in the series.