Complex sentence: The Gems soon meet Uncle Grandpa, but think that he is dangerous; as they prepare to attack Uncle Grandpa, he is able to summon a plot hole from Belly Bag, which transports himself and Steven to Uncle Grandpa's RV.
Split into simpler sentences: The Gems soon meet Uncle Grandpa, but think that he is dangerous. As the Gems prepare to attack Uncle Grandpa, he is able to summon a plot hole from Belly Bag, which transports himself and Steven to Uncle Grandpa's RV.