It's Saturday, the best morning of the week when a scream interrupts the perfect day for learning about butterflies. Mili the guppy is missing! But have no fear, P.I. Butterfly is on the case. She'll stop at nothing to solve the mystery. P.I. Butterfly preserves the scene, records every clue, and interviews suspicious suspects, but she ends up on a trail to nowhere fast. As she drowns her sorrows in a glass of milk, a fishy finding leads to a new suspect and a cat-felt confession.