Saki Watase is a 26-year-old Japanese office worker grappling with financial strain. Seeking extra income, she takes a part-time job as a product tester, unexpectedly assigned to review XL-size condoms—a challenge given her current lack of a romantic partner. Diligent, conscientious, and persistent, Saki rarely gives up easily. She enjoys alcohol despite a low tolerance, sometimes leading to heavy intoxication during work socials.

Professionally, she works under Keisuke Sudou, initially perceived as a strict, demanding superior she privately calls her "demon boss." After a company drinking event leaves her drunk, Sudou escorts her home, spots the XL condoms, and learns about her testing job. He reveals his own XL-size anatomy and volunteers to help. This initiates a romantic relationship between them.

Their relationship faces complications when the charismatic Riku Narata develops an interest in Saki, creating a love triangle. This introduces relational tension and emotional conflict as she navigates her feelings for both men. Her character development explores romantic and sexual dynamics within these relationships, particularly her evolving connection with Sudou. Their collaborative condom testing sparks mutual attraction and deepens their emotional bond.