Kirika-centric plot. Her brother appeared one day, and he wants to take Kirika away. He wants her to come live with him again just like old days. Kirika refused but he put a choker to her that will have 2 effects:
1. It's a timed bomb that will explode at dusk of that day. Also, it will blow off if forcefully taken off.
2. Can't remember the exact name of the curse, but it has the effect of intensifying the user's feeling of loneliness, so it's like Kirika will lose herself unless she constantly get in contact with Yachi.
He proposed a deal that if Kirika came round to him, her brother will take off the choker. The second option is to find the 50 stuff toys that her brother hid within the school. 1 of them contains the key and 1 of them is a trap that will blow the stuff toy itself.
Of course, Kirika and the gang refused (who would want to live with that freak brother who is fine with blowing off his little sister's neck?) so they chose the second option. They swear to find all the stuff toys around school. Due to time constraint they decide to split up individually so the search would be efficient, but it seems Kirika can't go alone without Haruaki... and I think you have the idea of the prospective events.
Sounds like a wtf plot at first glance but... I refute that. Just read it if you want to find out the rest.