Don and his team investigate a trail of illegal waste disposal after a sinkhole, which collapses in a school playground, kills an elderly woman and injures several children.

As the FBI tries to prove that a corrupt construction company is to blame because they have been burying containers of toxic waste under school playgrounds, they discover that the students in the school have a higher rate of cancer and neurological problems. Charlie tries to mathematically prove cause and effect between the pavement materials and the cancer clusters. Meanwhile, Millie, Charlie's new boss at Cal Sci, orders him to spend less time helping out the FBI and more time in their department.

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