The girls are waiting on tenterhooks to see whether their parents will agree to let Marcia have a slumber party. They are squealing with delight when the parents agree to it, and Cindy and Jan will be part of it too. Preparations, planning, and invitations are soon underway. Alice warns Mr Brady that they could be in for a chaotic time with the slumber party. Carol promised Marcia that she and Mike will stay out of the way at the slumber party, so they plan to go out for dinner, and leave Alice to hold down the fort.
The boys are not thrilled at the thought of being invaded by girls, or their sleeping bags being used for the party. But then Greg suggests they play pranks on the girls during the party, which will include rubber spiders, Halloween masks, and itching powder in the sleeping bags. The parents are surprised when the boys suddenly become more enthusiastic about helping with the party.
Then Mr Randolf the headmaster summons Marcia to his office. Marcia's teacher, Mrs Denton, found what looks like an unflattering caricature of herself and the caption "Mrs Denton or a hippopotamus?" Marcia admits to drawing the picture, but claims it is meant to be George Washington, and she knows nothing about the caption. Mr Randolf does not find Marcia's explanation satisfactory, much as he would like to believe her protests of innocence. He gives her a week of detentions. When the parents hear, they cancel Marcia's slumber party. Marcia is extremly upset that her parents do not believe her.
Mike goes to see Mr Randolf to investigate the matter. Mr Randolf says that on the basis of what he has, he cannot conclude anything else. Mike is not so sure. He asks for, and receives, the drawing. After examining it, Mike and Carol decide there is a reasonable doubt and a mistake was made. They cannot prove it to Mr Randolf, but they do reinstate the slumber party. Marcia is delighted to hear that her parents finally believe her. Mike is surprised when the boys react with delight to the news and being so eager to help the party along.
However, Marcia has been thinking as to who could have made the caption. She concludes it was her best friend Jenny Wilton because Jenny was the only one she knew who could have used her desk at the time. She phones Jenny and angrily tells her that she is no longer her best friend and uninvited to the slumber party.
The party is in full swing, and the parents are heading off to dinner. The boys strike in Halloween costumes during games of Truth or Dare and telling ghost stories. Alice becomes an unintentional victim when she finds a flashing skull in the refrigerator. Then the itching powder in the sleeping bags takes effect, and the girls are sent upstairs to wash it off.
Marcia is furious with her brothers, but her friend Paula says she can see the funny side as she likes jokes, such as the one they pulled on Mrs Denton. When Marcia presses Paula on this, Paula says she found the funny picture of what she thought was Mrs Denton and wrote the line below it. When Paula hears that Marcia was punished for it, she apologizes; she did not mean anyone to see it but Marcia, and it was just to give Marcia a laugh. She says she will explain to Mr Randolf.
The parents return. Marcia explains what she has just learnt and what she did to Jenny. She realizes she has made the same mistake as Mr Randolf and asks their advice on what to do. They advise her to phone Jenny, apologize, and invite her over. Jenny soon arrives. When she does, Mike becomes a victim of another prank - a bucket of flour over the door.
On Monday, Mr Randolf phones to say that Marcia and Paula have explained. Everything has been sorted out and forgiven. Alice brings over a box of cookies for Mike. When Mike opens it, he is pranked again when he finds a rubber spider in it.