According to Mushmouse's horoscope book, today is Punkin's unlucky day. Sure enough Punkin' encounters many misfortunes until the next day, which is Mushmouse's turn to face misfortune.
Mushmouse mixes a potion similar to Dr. Jekyll's formula, which turns him into a monstrous mouse. When Punkin' tries it, he unleashes a mean doppelganger of himself.
Mushmouse inherits from his late uncle £1000. Punkin' treats Mushmouse better in hope of getting a share. To their surprise the inheritance is one thousand pounds of cheese.
Punkin' Puss gets a knockout, dreaming of the Feudal Age with Saxon mice and their king Mushmouse. Punkin' is faced with Meany Mouse and eventually wakes up from this dream and resumes feuding with Mushmouse.
After Mushmouse gets Punkin' knocked out in a feud, he tricks him into thinking he's become a ghost. Mushmouse proceeds to get Punkin' to spoil him, until Punkin' sees through his trick.
Mushmouse orders a Mouse From S.O.M.P. Kit, so he can defend himself from Punkin'. Punkin' gets a similar kit to counter Mushmouse getting them evenly matched.