As a Young Scientist, twelve-year-old Addie studies pollution in the lake where her twin brother recently drowned, while secretly continuing his investigation of the creature he believed lives in the lake's depths.

Obe Devlin spends a lot of his time cleaning up the creek that runs through what little is left of his family's once extensive farmland, and worrying about what the developers are doing nearby, and the pollution it is causing--but one day he finds a strange creature by his creek that eats plastic, and soon the animal he calls Marvin Gardens becomes his personal secret, which he believes needs to be protected from pretty much everybody.

When the lake in one of Fogville's parks suddenly becomes polluted and starts to boil, Echo the bat and his human friends, the Bat Pack, venture into the sewers to find the monster that may be causing the problem.