The 'Call to Adventure' occurs in the second stage of the hero's journey or adventure. Equally important to understand, the 'Call to Adventure' is usually delivered by a character known as a herald. The herald usually bears some sort of life-changing, or threatening, news to the hero, and upon receiving such news, the hero's life will never be the same again.
In Tolkien's novel, The Fellowship of the Ring, Gandalf, in the role of the herald, imparts the news to Frodo that his Uncle Bilbo's ring may actually be the One Ring, Sauron's long lost ring of power and ultimate evil. The moment that Gandalf tosses the ring into the fireplace at Bag End, the call to adventure begins; for in that moment, Frodo begins to realize that he has a greater part to play in events that will shape the future of Middle Earth. The hobbit admits to Gandalf that he had "sometimes thought of going away" like a "kind of a holiday [...] but this would mean exile, a flight from danger into danger" (61). Frodo knows that delivering the Ring to Rivendell will prove extremely dangerous, but even though he feels "very small" and not like a hero at all, he reluctantly accepts the call (61).