Because she thinks her own children do not appreciate her anymore, Marge becomes a mother figure for Nelson.
Feeling that her own kids take her for granted,Marge takes pity on Nelson,whose dad has walked out on him and his loose-living mother,and lets him stay at her house. Bart is horrified as it means he gets bullied at home as well as at school. There is only one thing for it. Bart goes out and finds Nelson's father,bringing happy outcomes all round.—don @ minifie-1