The moment in the Hero's Journey where the protagonist is invited to leave their ordinary world and enter the unknown.
The call to adventure is the beat in Joseph Campbell's Hero's Journey where something or someone disrupts the protagonist's normal life and presents them with a challenge, a quest, or a problem that demands they leave their comfort zone. It might come as a literal invitation, a desperate plea for help, a mysterious discovery, or a threat that cannot be ignored. The key is that it offers the hero a choice: step into the unknown or stay safe. Most heroes, at least initially, choose to stay safe.
The call to adventure does two things at once. It launches your plot by giving the protagonist a direction, and it reveals character by showing how they respond to disruption. A character who leaps at the call is fundamentally different from one who hides under the bed. How your hero responds to this moment tells the reader everything they need to know about who this person is at the start of the story, and sets up who they might become by the end.
Gandalf reveals the true nature of Bilbo's ring and tells Frodo it must be destroyed. The call is delivered quietly in a hobbit hole, but the weight of it is enormous. Frodo does not want to go. He goes anyway.
If your hero immediately says yes to every adventure, they feel reckless rather than brave. The refusal of the call, even a brief one, shows the reader what the hero stands to lose and what they are afraid of. That makes saying yes meaningful.
The hero needs to understand what they are being asked to do, even if they do not understand the full scope yet. 'Something bad is happening' is not a call. 'Your village will be destroyed in three days unless someone reaches the mountain' is a call.
They often overlap, but they are not identical. The inciting incident is the disruption. The call to adventure is the invitation to respond. In some stories they are the same beat; in others, the inciting incident happens first and the call follows.
Write the same call to adventure three different ways: once as a herald delivering news, once as a discovery the hero stumbles upon, and once as a threat that forces their hand. Notice how each version changes the hero's emotional response and the tone of the story. Pick the one that makes the hero's eventual yes feel most earned.