One summer morning, Annie and her brother Bradley venture down to a nearby stream to play, but find the water murky and devoid of frogs and minnows. They hear a splashing sound behind them, and are startled to see their neighbor, Ms. Carson, approaching them, wearing rubber boots and carrying a box of test tubes. It turns out Ms. Carson is an aquatic ecologist whose job it is to test the health of local streams. After Annie mentions all the green slime found in the stream, Ms. Carson explains that it is the result of a build-up of algae caused by excessive amounts of nutrients flowing into the stream. Ms. Carlson and the kids then gather some water samples in test tubes, to which they add special chemicals that reveal that the water contains excessive amounts of phosphorous and nitrogen.

Ms. Carlson explains that these are derived from fertilizer and pesticides that their neighbors have been using to excess. Upon hearing this news, Annie and Bradley begin a special campaign to get their neighbors to reduce the amount of chemicals they use in their lawns and gardens, which results in a healthier stream the following year.