It's a hodge-podge of Terrytoon plots from the previous decade and a half as people sing operetta while the wolf is at the door threatening to foreclose on the mortgage unless the grand daughter will marry him; her lover, however, will win the Kentucky Derby with the help of Mighty Mouse, paying 18:5 on the pari-mutuel board, but hold your tickets while they conduct an enquiry.
All in all, it looks like Paul Terry's writers rolled their eyes and started throwing plot points against the wall while Script Editor John Foster looked on aghast, then noted that the audience of small children seemed to like anything with Mighty Mouse in it, so why not?
Did anyone on the staff even care by this point? Oh, well. At least he's not beating up on cats yet again.