What sets Indiana Jones on his adventure?

When Indiana is back to San Francisco, two Army Intelligence agents come to him with information that the Nazis are looking for his mentor Abner Ravenwood who was an expert in Egyptian archaeology and owned an artifact called the Staff of Ra. In the letter overtaken from the Nazis the city of Tanis is mentioned, so Indiana assumes that the Nazis are searching for the Ark of the Covenant – the most precious artifact in the world. It is believed to have power of God, and the one who would possess it would be invincible.