What I think is especially compelling about the story of the Silver Surfer is that it is presented in layers. He was introduced in Marvel Comic’s Fantastic Four title, issue 48: “The Coming of Galactus!”
The Call To Adventure is not for the Silver Surfer, but is externalized to the Fantastic Four. (The internal call will be explained in the Surfer’s origin story, where Norrin Radd becomes The Hero by way of Galactus.) They are squarely in the realm of the Known, while the Surfer is an Unknown.
The threat, as is discovered later, is Earth’s impending destruction, to feed the hunger of a cosmic being called Galactus.