Riding with Isabel, the pair manage to slip past or get around some of the official roadblocks intended to keep the populace in place. That is, until they reach the checkpoint at 495. Now, the Merrimack River becomes a barrier. Once again on foot, they follow the railroad tracks toward Haverhill. Martin hopes an interstate bridge over the river will be less guarded. Before they get there, the sky grows dark and it begins to rain.