Dawn thought having a stepsister would be fun. Was she ever wrong!
Dawn's mom and Mary Anne's dad have just got married and Mary Anne catches the flowers thrown by Sharon, Dawn's mom.
Dawn is envious of Mary Anne for catching the flowers instead of Dawn, thinking that she should have, because it was her mother's wedding but soon Dawn calls Mary Anne sis and things are fine. for a while.....
Later Dawn and Mary Anne struggle to be best friends when they have trouble getting along such as when Mary Anne's dad Richard makes breakfast he serves Sharon bacon and Sharon can't stand bacon but she says she likes whatever he feeds her for breakfast. Later at dinner the same problem goes on when Sharon makes tofu and Mary Anne and Richard do not eat tofu.
Richard also declares spring cleaning while he is very organized. Sharon is a scatterbrain, so Mary Anne pickes up after Sharon, which irritates Dawn.
To drive Richard crazy, Dawn messes up his sock drawer, only Richard thinks it was his falt for the mishap.
Much later Dawn and Mary Anne are furious with one another after the events going on in the Schafer-Spier household such as Sharon being disturbed about Tigger, Mary Anne's cat, throwing up on the oriental rug.
Mary Anne starts spending more and more time at Kristy's house during the family fight, but does not bother Dawn.
Finally Dawn pretends to be old Jared Mullray to scare Mary Anne into switching rooms and making up her plan works, and they are friends again.
Meanwhile, the "Pike plague" is going on at the pikes when the whole family gets sick including Mr. and Mrs. Pike.