Mallory hasn't been feeling well for awhile now. And finally she discovers why she's been so tired and run-down. Mallory has mono.
Having mono means staying in bed and resting - for a long time. No school, no activities, no baby-sitting, and no Baby-sitters Club.

Mallory is crushed. So are the Baby-sitters. It's bad enough that Dawn is away in California. But now it looks like Mallory's out of the club, too.