Complex sentence: The plot of "Riddick" will find the titular criminal once more on the run from mercenaries who want the hefty reward his hide will bring who once again ends up on a planet teeming with unfriendly alien beasts.
Split into simpler sentences: The plot of "Riddick" will find the titular criminal once again on the run from mercenaries after the bounty placed on Riddick. At some point in the film, Riddick will once again find himself on a planet teeming with unfriendly alien beasts.