Rather than a detailed plan, I prefer to see a strong vision, a strategy, goals, and a roadmap (high level outline plan). The tactics to achieve this, for example the precise features and all the tasks to deliver them, can vary along the way and are best not articulated up-front. This enables the team to discover the details when they are in a better position to do so, and allows them to change direction rapidly in response to changing circumstances.
Just read this article on the problem with software planning. I think this paragraph really hits the nail on the head regarding the benefit of flexibility in an agile software development environment.
