"Plus-Sized Elf" centers on Tomoatsu Naoe, a massage therapist with a knack for treating unusual clients. One evening, an elf named Elfuda, or Elfo for short, stumbles into his clinic. Elfo, a visitor from a fantasy world, has become overweight after indulging in human food and seeks Naoe's help to shed the extra weight. Despite her initial reluctance, Naoe's expertise in massage and health advice gradually helps Elfo, though her love for junk food and lack of discipline often complicate the process.

The series introduces other fantasy characters who face similar struggles after adapting to human lifestyles. These include a dark elf, a mermaid, and a dragon girl, each dealing with their own quirks and challenges. Naoe becomes their go-to advisor, offering practical solutions while navigating their unique personalities and cultural differences. The interactions between Naoe and these characters form the core of the narrative, blending humor, lighthearted moments, and occasional insights into health and self-care.

Elfo, the primary focus, is portrayed as confident and unapologetic about her size, though she occasionally expresses frustration with her inability to return to her original form. Her dynamic with Naoe evolves from a professional relationship to a friendship, with Naoe often acting as a supportive figure. The series balances episodic storytelling with ongoing character development, exploring themes of body positivity, cultural adaptation, and the challenges of maintaining health in a world filled with temptations. The tone remains light and comedic, with occasional moments of sincerity that highlight the characters' growth and relationships.