Is your audience receptive to what you have to say? are they curious and inclined to believe you? Tell them your conclusions right up front, so if they don't hear anything else you say, they learned what they need to know. It will also help focus their attention as you proceed.
If instead they are apt to push back on what you plan to say (because it would eliminate their jobs/research for example), then instead you start by building a case with a series of steps they will agree with till, having led them down that primrose path, you reveal to your new allies the inescapable conclusion. They'll still push back, but they will at least have learned what you have to say.