Jerry Mouse is a friend of you and Nibbles, minus Tom Cat, during Halloween night, you went to Frankenmouse's castle and to a surprise, no one is home, you and Jerry went to look around to see if Dr. Frankenmouse is home or not, but Jerry got exhausted from the search and went to find a drink to gain his energy back, until he came across a green potion on the counter, when he drank it without realizing it, Jerry Mouse became Monster Jerry by surprise. If he sustains enough damage or being hit with the green potion, he will revert to his normal self. However, Jerry can change back into Monster Jerry by either drinking the green potion or going berserk.