Complex sentence: The plan was that after her death they would divide the money in three equal parts and move on, but the plan begins to go awry when the detective on duty in the penthouse senses something fishy about Sheila dressing up as someone else leaving the penthouse.
Split into simpler sentences: The plan was that after her death they would divide the money in three equal parts and move on. But the plan begins to go awry when the detective on duty in the penthouse senses something fishy about Sheila dressing up as someone else leaving the penthouse.