Complex sentence: The twins view a video of the Wells Experiment, a 1930s time travel project that resulted in a boy being frozen in time, and which reveals the nature of the Chronos Sapphire in Rebecca's necklace.
Split into simpler sentences: The twins view a video of a time travel experiment from the 1930s called the Wells Experiment, that resulted in a boy being frozen in time. The video reveals to the twins the nature of that the Chronos Sapphire in Rebecca's necklace.