In scriptwriting, a catalyst is a character, event, or set of circumstances that motivate or otherwise cause a protagonist to take action, start a quest, or a journey.
The catalyst is almost always something bad, difficult, mysterious, or tragic in the storyline that catalyzes the protagonist to go into motion: it is the event that starts the protagonist on the path toward the conflict. In Star Wars: Episode IV – A New Hope, Luke sees the tail end of the hologram of Princess Leia, which begins a sequence of events that culminate in plot point I.
Also known as the inciting incident, the catalyst is the moment where the plot begins taking its shape. It is the moment where the protagonist, a character on a quest, is first exposed to the bad thing that will ultimately drive the protagonist to the conflict in the story.