When an explorer’s ancient necklace is stolen, she must retrieve it before it is used for evil.
Your logline is short enough you could add a few details to flesh out the story in the reader’s mind:
When her ancient necklace (That can control the weather) is stolen by an evil (Witchdoctor) an intrepid explorer with a knack for trouble must get the artifact back, before it’s used to (wreak havoc on the world)
Who is “she”? Give us more information about your protagonist.
You haven’t specified that the necklace is magical in any way… maybe confirm this just to dispel any doubts and give us more specifics about how it is actually going to be used for evil. This way, we understand what’s at stake.
As mikepedley 85 said: what is so magical about the necklace? And who stole it? In “Raiders of the Lost Ark” the power of Ark of the Covenant to destroy armies is part of the legend. And the villains who want to aquire the Ark to exploit that power are Nazis. So the stakes are self-evident and ultra-high and Indiana Jones is highly motivated.