The Chemical Substance of Love (S02E09)

The Beppu Brothers declare outright to the Defense Club that they're the ones who have been sending the monsters, but the Defense Club members just brush them off, spending their days in their usual slow pace while ignoring the brothers for two whole months. The Beppu Brothers, enraged over this slight, choose February 14th as the day they will have their revenge. And so the Valentine's Day grudge match begins! (Crunchyroll)