Sixteen year-old Shur is one of the first to see the creatures. It looks like a monarch butterfly—but it's huge. It's only when she's left school and headed home that she sees what the creatures do: they attack. By the time Shur, her two brothers, and their two best friends make it back to her house, it's clear the five of them must survive whatever comes next on their own. As the "butterflies" multiply and swarm outside, anxiety-prone Shur focuses on what she can control. Meanwhile the creatures begin to fulfill their ultimate purpose: multiplying via parasitic load, and before long, the butterflies aren't the only thing trying to get in. To protect her family and survive the invasions, Shur must find the strength to protect their sanctuary at any cost.