The Secret Guardians

The Secret Guardians is the second half of the 7th episode of Season 13. It first aired on April 6, 2010 with The Portrait of the Artist as a Young Tibble.

Summary
Arthur, Binky, and Sue Ellen find a woodland oasis near their homes and vow to keep the area a secret. Soon they start to find evidence of human habitation and wonder if they've discovered an ancient civilization.