Six middle-school age girls from varying backgrounds get themselves trapped together in a large underground outfall connected to an abandoned treatment plant for the local sewer system.
They need to find their way out in two hours before the next water discharge floods the tunnel. They have to travel from the diffuser tunnel to the outfall tunnel, or approximately two miles under sea level, in order to reach the other exit located near the bottom of the outfall shaft, one hundred feet under water.
Along the way, they find a bag filled with money and the inscription "Millborough Savings Bank" along the side. But they can't tell any adult because, while down there, anyone with a cell phone finds that their devices aren't working sufficiently, thus they cannot call for help letting anyone know they're trapped.
However one text message is received and comes through on a cell phone VERY clear: "I'm coming for you!"