When he finds out who killed his father, a laid-back youngster must return to save everyone from his evil Uncle and become the rightful king
The plot of this story seams to resemble Hamlet or the Lion King and I suppose if adapted to humans, in live action and present day it could be interesting.
What period is this story set in and where?
I would also change the wording to sound more dramatic and motivating than it is now. I find the father’s murder more compelling of an event than the discovery of the murderer’s identity:
After his uncle kills his father a rebellious prince who rejects the monarchy must become king in order to save his family from his uncle.
Hope this helps.
In terms of content, if you’re deliberately writing a modern twist on Hamlet, more specific details of your setting are needed to help sell it – “laid back” doesn’t really give a clear description of the protagonist. Is he a college student? Does he work a dead-end job? Is he in a band? And King of what?
In terms of structure, there is a passiveness to your logline which makes it a bit dry. Perhaps something like this would spruce it up? “After the murder of his father, a young prince must return home to confront his power-hungry uncle and take his rightful place as king.” It still isn’t great but it is slightly less wordy with more immediate action.