"Isekai Onsen Paradise" is a fantasy-comedy anime set in a world where hot springs hold mystical properties. The protagonist, Haruto, a modern-day office worker, is transported to a magical realm after falling into a hot spring during a company retreat. This world, known as Therma, is governed by the power of onsen, which grant unique abilities to those who bathe in them. Haruto discovers he has been chosen as the "Onsen Sage," a legendary figure destined to restore balance to Therma by reviving its dying hot springs.

The narrative revolves around Haruto’s journey to locate and rejuvenate the sacred springs scattered across Therma. Along the way, he is joined by a diverse group of companions, each with their own motivations and connections to the onsen. Among them is Sora, a skilled warrior who protects the springs, and Yuzu, a mischievous spirit who serves as a guide. The group faces various challenges, including rival factions seeking to exploit the springs for their own gain and ancient curses that threaten to destroy Therma’s natural balance.

The series blends episodic adventures with an overarching plot, as Haruto and his companions travel to different regions, each with its own unique hot spring and associated lore. The narrative explores themes of environmental preservation, cultural heritage, and personal growth, as Haruto learns to embrace his role and the responsibilities it entails. The interactions between the characters provide both comedic moments and emotional depth, highlighting their evolving relationships and individual struggles.

As the story progresses, Haruto uncovers the truth behind Therma’s decline, tied to a forgotten conflict between humans and spirits. The climax involves a confrontation with the antagonist, a former Onsen Sage who seeks to harness the springs’ power for domination. Through teamwork and understanding, Haruto and his allies restore harmony to Therma, ensuring the survival of its magical onsen and the world they sustain. The series concludes with Haruto facing a choice: return to his original world or remain in Therma as its protector.