Kagami ni šiteru mitai de ija nande kedo (S03E01)

The grand event for elementary schoolers is finally here--Summer Break! In preparation for the long-awaited trip to the beach, Illya and the others go with their classmates to pick out new swimsuits and carry on in front of the skimpy clothing♪ While they're out, Kuro runs out of mana and calls Illya away to an empty stairway platform. But Mimi catches sight of them in the act?! (Crunchyroll)