When Mosca and Clent attempt to cross a treacherous river in the mountains, they get stuck in the town of Toll, who has a split personality hinging on day and night. Fly Trap features the young girl called Mosca Mye, who is accompanying her mentor, Eponymous Clent, across the mountains to seek refuge for the winter. In the previous book, the duo had accidentally-on-purpose helped foster a rebellion in the biggest port city in the land. It is not advisable to read this book if the book preceding it, Fly by Night, has not been read, many things will be unclear for those who do. I found this book interesting and a good read, especially as a sequel. Mosca and Eponymous, like most of the rest of the town's residents, scramble to come up with enough money to escape Toll, which is easier said than done. The terrifying Guild of Locksmiths already control half the town and are close to seizing control of the other half. Anyone caught breaking curfew is never seen again. Half the people are locked into their homes during the night, while the other half are locked up during the day, and the two groups never meet. The town's residents are divided into day and night based on their names. Once inside Toll, Mosca and Eponymous discover that the pretty cobble-stoned streets hide a rotten core. Unfortunately, Mosca had stolen enough money to get them into the town but not enough to exit on the other side.

They end up in the town of Toll, built around the only bridge over the River Langfeather and where people must pay a toll to get into the town and another to leave. Mosca, her goose Saracen, and her employer Eponymous Clent are on the run again.

Fly trap by frances hardinge