Mary Anne should never have thrown away that chain letter she got in the mail. Ever since she did, bad things have been happening — to everybody in the Baby-sitters Club. With Halloween coming up, Mary Anne's even more worried — what kind of spooky thing will happen next?
Then Mary Anne finds a new note in her mailbox: Wear this bad-luck charm, it says. OR ELSE. Mary Anne's got to do what the note says. But who sent the charm? And why did this person send it to Mary Anne?
If the Baby-sitters don't solve this mystery soon, their bad luck might never stop!
Set before Halloween, Mary Anne gets a chain letter in the mail which orders her to send out 20 copies of the letter or not only she would have bad luck, but her friends and family will too. After Mary Anne ignores the letter, bad things start happening to her & her friends.
A few days later, Mary Anne gets a necklace in the mail which orders her to wear the bad luck charm. Even though Mary Anne doesn't want to, the rest of the BSC thinks that she should. The BSC tries to find a good luck spell, but to no avail.
After Mary Anne attends her school's Halloween Hop with Logan, she finds a note taped to her door telling her to appear at Old Hickory's grave at the cemetery on Halloween at midnight. After calling the rest of the BSC, everyone decides to go for it (with Kristy using a late night slumber party as a cover).
In the end, it is discovered that Cokie Mason was responsible for the whole thing, so the BSC decide to get back at Cokie & her friends. The BSC goes to the cemetery, rigs up some bedsheet ghosts with a sound effects tape, flashlights, and scary masks for effect. When Cokie and her friends arrive, the BSC scares them.
This book mentions that Mary Anne and Dawn watched Night of the Living Dead and the former also watched Halloween and Halloween II.
A reprint of the book will be released August 3, 2021 
Ann M. Martin's Dear Reader Letter
Mary Anne’s Bad-Luck Mystery is the second spooky book in the Baby-sitters Club series, and the first book that takes us to Old Hickory’s grave in Stoneybrook’s cemetery. When I was young, I was fascinated by graveyards. And like the rest of my friends, I believed in a superstition about them — that I must hold my breath if I went by a graveyard. For this very reason, when my family was on long car trips, my father would often park our car in front of a graveyard. Then my sister and I would try to hold our breaths for as long as we were parked there!
The idea for this story, however, came to me after I received a chain letter in the mail. It was not an eerie chain letter like the one Mary Anne receives. It was just a regular one. But I hate chain letters, and began to wonder if this would make a good plot for a mystery. I think it did. And later, it led to The Baby-sitters Club Chain Letter book, which is all about an unusual chain letter that is passed through the members of the BSC.