Twins Iris and Lark have been inseparable from their first moments in the world together--Iris the grounded, rational, capable one, Lark the dreamy, intelligent, and creative one. When fifth grade rolls around, however, it is decided the twins should be split up to separate classes--and something splits inside both. Lark withdraws into herself and Iris loses her self-confidence. At the same time, things mysteriously begin vanishing around the city, and Iris takes it upon herself to keep Lark safe.