"Jikuu Tantei Genshi-kun" is a manga series that blends elements of science fiction, mystery, and adventure. The narrative centers around Genshi-kun, a young boy who is a time-traveling detective from the prehistoric era. Accompanied by his dinosaur companion, Ptera, Genshi-kun travels through various time periods to solve mysteries and prevent disruptions in the timeline caused by an enigmatic villain known as Doctor Z.

The story begins with Genshi-kun being summoned from the prehistoric past to the present day by a scientist named Professor Tange. The professor has developed a time machine and enlists Genshi-kun's help to investigate anomalies in history. Genshi-kun's unique perspective and primitive instincts often provide unconventional solutions to the problems he encounters. Each arc typically involves Genshi-kun and Ptera traveling to a different historical period, where they interact with notable figures and events, uncovering the truth behind the disturbances.

Doctor Z, the primary antagonist, is a shadowy figure who manipulates time for his own gain. His schemes often involve altering historical events or creating paradoxes, which Genshi-kun must unravel and correct. The dynamic between Genshi-kun and Doctor Z drives much of the series' tension, as their confrontations span across different eras and settings.

Supporting characters include Professor Tange, who provides scientific expertise and guidance, and various historical figures who play roles in the episodic mysteries. The series explores themes of history, causality, and the consequences of tampering with time, all while maintaining a lighthearted and adventurous tone.

The episodic structure of the manga allows for a wide range of settings and scenarios, from ancient civilizations to futuristic worlds. Each case Genshi-kun solves contributes to the overarching narrative of thwarting Doctor Z's plans and preserving the integrity of the timeline. The interplay between the prehistoric protagonist and the advanced technology of time travel creates a unique juxtaposition that adds depth to the storytelling.