"Senyū" is a comedic fantasy manga series set in a world where humans and demons are locked in an ongoing war. The story centers on Alba, a young and inexperienced hero, and his companion, the demon princess Ross, who is bound to serve him due to a magical contract. Alba is part of the Hero Clan, a group tasked with defeating the Demon King and ending the war. Despite his lineage, Alba is far from the typical hero—he is cowardly, lazy, and often relies on Ross to handle dangerous situations. Ross, on the other hand, is powerful and pragmatic, frequently exasperated by Alba's incompetence but bound to protect him.

The narrative unfolds through episodic adventures as Alba and Ross encounter various demons, allies, and enemies. Their journey is marked by absurd humor, often subverting traditional fantasy tropes. Alba's lack of conventional heroism is a recurring theme, as he frequently avoids direct combat and relies on unorthodox methods to survive. Ross, despite her strength, is often forced to clean up the messes Alba creates, creating a dynamic of comedic tension between the two.

Other key characters include Sheila, a skilled warrior and childhood friend of Alba, who often contrasts his ineptitude with her competence and bravery. The Demon King, though a looming threat, is portrayed in a humorous light, with his plans frequently undermined by the incompetence of his subordinates. The series also features a variety of eccentric side characters, including other heroes, demons, and magical beings, each contributing to the chaotic and unpredictable nature of the story.

The plot is driven by a mix of slapstick comedy, satirical elements, and occasional moments of genuine peril. While the overarching goal is to defeat the Demon King, the focus remains on the comedic interactions and misadventures of the characters. The series balances its humor with occasional glimpses into the darker aspects of the war, such as the moral ambiguity of both sides and the personal struggles of the characters. Despite its lighthearted tone, "Senyū" occasionally delves into themes of loyalty, duty, and the complexities of heroism, though these are always framed within its comedic context.