"Tanaka-kun wa Itsumo Kedaruge" centers on Tanaka, a high school student whose defining trait is his overwhelming lethargy. Tanaka strives to live a life of minimal effort, often falling asleep in class or relying on others to handle even the simplest tasks. His best friend, Ohta, is the polar opposite—physically strong, dependable, and perpetually tasked with carrying Tanaka around when he becomes too tired to move. Despite their differences, the two share a close bond, with Ohta often going out of his way to accommodate Tanaka's laid-back lifestyle.

The series unfolds in a slice-of-life format, focusing on the daily interactions and comedic situations that arise from Tanaka's extreme laziness and the reactions of those around him. The narrative introduces a variety of characters who each bring their own quirks and dynamics to the story. Miyano, a bubbly and energetic underclassman, idolizes Tanaka for his calm demeanor and seeks to emulate his "listlessness," though her efforts often result in humorous misunderstandings. Echizen, a stoic and athletic classmate, becomes an unexpected friend to Tanaka, while Shiraishi, a beautiful and popular girl, develops a crush on him, leading to lighthearted romantic tension.

Throughout the series, Tanaka's interactions with his peers highlight the contrast between his desire for a quiet, uneventful life and the chaos that naturally surrounds him. Despite his constant attempts to avoid exertion, Tanaka inadvertently becomes a central figure in the lives of his friends, who find his nonchalant attitude both baffling and endearing. The story maintains a lighthearted tone, with humor derived from Tanaka's exaggerated laziness and the ways in which his friends adapt to his peculiar habits.

The episodic structure of the series allows for a focus on character development and relationships, with each episode presenting new scenarios that test Tanaka's resolve to remain inactive. Whether it's participating in school festivals, dealing with misunderstandings, or navigating social dynamics, Tanaka's journey is one of comedic misadventures and subtle growth, all while maintaining his core philosophy of avoiding unnecessary effort. The series balances its humor with moments of genuine warmth, showcasing the bonds of friendship and the acceptance of individuality within the group.