Arnold first discovers his father's old journal in the attic
The journal is the object that changed Arnold's life forever. Originally owned by his father, the stories in the journal helped Arnold learn more about his long-lost parents. The journal also contains a map of the San Lorenzo jungle. In Hey Arnold!: The Jungle Movie, the journal and map play significant roles as the keys to Arnold finally finding his parents.
Background[]
Approximately ten years before the main events of the series, Miles Shortman started keeping the journal when he began living and doing research work in San Lorenzo. Miles used his journal to detail all of the adventures and major events he experienced while living in the country. Stories included when he first met Stella, the couple's first encounter with the Green-Eyed People, their wedding day, their making of the serum for the sleeping sickness-stricken Green-Eyes, and the volcanic eruption that happened during the birth of their child, Arnold.
After Arnold's birth, the family left San Lorenzo and moved back to Hillwood, though Miles occasionally continued to write in his journal. His last entry was about his and Stella's reluctant decision to temporarily leave Arnold and return to San Lorenzo to help the Green-Eyes again after Eduardo tells them about another sleeping sickness outbreak.