In a recent Entertainment Weekly interview, Steven Spielberg revealed that George Lucas has figured out the genre of Indiana Jones 5, and that they have a concept in mind, but he doesn't know where the story is at.

"It's up to George. We have already agreed on the genre of the fifth movie, we already have a concept in mind. I don't know where George is with the story. There is no Indiana Jones 5 until George says there is."
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George Lucas has now come forward to admit that the story has him stumped. While he has figured out a genre and a concept for Indiana Jones 5, he has not yet been able to find the perfect MacGuffin, the device (such as the Ark of the Covenent or the Crystal Skull) upon which to hinge the plot upon. And it may be a while before he settles on one.

"I really haven't found the MacGuffin yet. I mean, I know what it's about, but I just have to find a MacGuffin that fits into the arena we're working in."

When asked if Henry Jones, Jr.'s extended family would be playing a part in the story, George Lucas only had this to say.

"Yeah, we'll see!"