Cathedral tells the story of the battle to save the hemlock trees of Cook Forest State Park, Pennsylvania, which are under attack by the Hemlock Woolly Adelgid (HWA), a destructive insect that has already killed thousands of trees in the eastern United States. The film takes the viewer on a journey through the forest in all seasons and shows the important work being done by the Pennsylvania Department of Conservation and Natural Resources and the Pennsylvania Bureau of Forestry. Cathedral talks about the history of Cook Forest State Park, which is a registered National Natural Landmark, and explains how the hemlock tree is a keystone species, influencing everything from water temperature to bird populations. Cook Forest is home to the one of the last, best remaining stands of old-growth forest in the United States, and is considered by many to contain the best remaining old-growth hemlock.