"Hollow City" begins where "Miss Peregrine's Home for Peculiar Children" left off. Jacob and the ten peculiar children have successfully fought off their first hollowgast and are afloat in three small row boats trying to cross the English Channel. Their small island refuge is no longer safe for them to live. They have Miss Peregrine, their head mistress, stowed away in the boat, hoping to find another ymbryne who can help her transform herself from bird form back into human form, but the dark forces are in close pursuit. World War II is ravaging England and the children must avoid bombs as well as hollowgasts as they search for the last remaining ymbryne to avoid capture.