Tomo did something stupid/lost a bet/etc, and now she has to do this as a punishment. All you have to be aware of is that Tomo is the type of person likely to make stupid bets where she would have to have a container of natto poured over her face if she lost.
Incidentally, I heard once that natto is sort of one of the grossest things ever (it's an acquired taste for people who eat it), and some girl-bullies/gang pour it on other girls as the equivalent of swirlies -- "sticking heads in toilets and flushing".
Chiyo:
Um... are you sure it's alright to be doing that?Osaka:
It's ok! Tomo-chan won't be defeated like this!Yomi:
Eat every last bite!Kagura:
Tomo! Hang in there!
