The narrative supports the slides and the slides support the narrative
Recently a colleague asked me to look over his slides. After reviewing this highly dense set of data and text slides, I told my friend not to "over think" the slides and concentrate on the overall message first. I drafted a couple of paragraphs (in Word, not PowerPoint) summarizing a potential narrative for the presentation.
My friend thought the narrative was helpful but didn't see the connection between the narrative and the data slides. He thought he could summarize the narrative on the first slide and then dive back into all the dense data and text slides. What he failed to see was that the narrative supports the data and the data supports the narrative.
It's fine to summarize the narrative right up front, but the subsequent data should support that narrative. The speaker needs to reveal the connection between the data and the narrative on every slide. The narrative makes each piece of data relevant to a bigger story. The data lends credibility to the narrative. Data that doesn't support the narrative shouldn't be presented.