Complex sentence: After Cassie hangs up, she asks Joseph to try and remember exactly what happened in 1968 at the Hampton Street Depository, as he was talking about his big secret that very morning.
Split into simpler sentences: After Cassie hangs up, she asks Joseph to try and remember exactly what happened in 1968 at the Hampton Street Depository. She reminds him that he was talking about his big secret relating to it that very morning.