Mariru is a modern girl with stylish inner hair color and a pure, innocent vibe. This documentary-style video follows her transformation from a girl uninterested in diapers or wetting into a full-fledged babygirl, guided by an older "childhood regression navigator" sister figure who uses gentle persuasion, pleading, and hypnosis to gradually break down her inhibitions. While sipping tea and answering questions about bodily functions, Mariru can no longer hold her bladder and ends up wetting herself while fully clothed. She's then forced to change into childish underwear—and soon wets herself again. After her sister remarks, "Adult wetting is exciting, especially when mixed with embarrassment," something other than urine begins to stain her panties. Her mental age regresses further, and she's now diapered in a paper diaper—only to wet herself once more. During cleanup, a slippery, non-urine substance is discovered, earning her a spanking on the butt. Giving up on denying her arousal, Mariru masturbates while still in her diaper. She then face-sits on her beloved stuffed toy, rubbing her crotch vigorously until she climaxes and wets herself uncontrollably. Now completely submissive, Mariru is dressed in cloth diapers. Though she's never seen them before, she finds the warmth comforting and enjoys the soothing scent of baby powder. Snuggling up to her "mommy" sister, she pees contentedly into the cloth diaper—her final evolution into a true adult baby.