"My Roommate is a Cat" centers on Subaru Mikazuki, a reclusive mystery novelist who struggles with social interactions and prefers solitude. His life takes an unexpected turn when he encounters a stray cat during a visit to his parents' graves. Moved by the cat's presence, he decides to adopt her, naming her Haru. The series alternates between Subaru's perspective and Haru's, offering insight into their evolving relationship and the ways they influence each other's lives.

Subaru, initially withdrawn and focused solely on his writing, begins to open up as he cares for Haru. Her playful and curious nature forces him to engage with the world outside his comfort zone, leading to gradual personal growth. Haru, on the other hand, transitions from a life of survival on the streets to one of security and affection, though she retains her independent spirit. The dual perspectives highlight their mutual dependence and the subtle ways they communicate despite their differences.

The narrative explores themes of companionship, healing, and the impact of small, everyday interactions. Subaru's interactions with Haru lead him to reconnect with people around him, including his editor, Nana, and his childhood friend, Hiroto. These relationships deepen as Subaru learns to navigate his emotions and responsibilities. Haru's perspective provides a whimsical yet poignant counterpoint, emphasizing her loyalty and the bond she forms with Subaru.

The series balances lighthearted moments with emotional depth, portraying the challenges and rewards of caring for a pet. Subaru's journey from isolation to connection is mirrored in Haru's transition from a stray to a beloved companion. Through its dual narrative structure, "My Roommate is a Cat" effectively captures the transformative power of companionship and the quiet joys of shared lives.