The CALL TO Adventure.
The hero is presented with a problem, challenge or adventure. Maybe the land is dying, as in the King Arthur stories about the search for the Grail. In Star WARS, it’s Princess Leia’s holographic message to Obi Wan Kenobi, who then asks Luke to join the quest. In detective stories, it’s the hero being offered a new case. In romantic comedies it could be the first sight of that special but annoying someone the hero or heroine will be pursuing/sparring with.