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roll up summary from formula field

My objects Project and Plan are master objects to "All projects" which is the junction object. I also have an object called "Activity" that is a child to Project and a lookup to the All Projects Junction object. I ultimately want a time field on Activity to roll up to Plan. Can anyone help? 
While formal Rollup Summary fields can only be created declaratively on child objects on a Master Detail relationship, you can programmatically create rollups between objects in a Lookup relationship. See Andrew Fawcett's recent BLOG post on a mechanism he has published to accomplish what you may be looking for: http://andyinthecloud.com/2014/02/09/new-release-spring14-declarative-rollup-summary-tool/.  Also see other related GitHub code that allows you to use his core class, and he even has some video on the topic: https://www.youtube.com/watch?v=6BST-TpyHHk