This is an official bonus map for Age of Empires III: The War Chiefs.
Plymouth is a festive new custom map from Ensemble Studios. It is a veritable cornucopia of Pilgrims, pumpkins, and envoys.
The latest patch (v1.01 or greater) for Age of Empires III: The WarChiefs is required for this map to run properly.
Download the zip file and then simply extract the contents (plymouth.xml and plymouth.xs) to your My Documents\My Games\Age of Empires 3\RM2 folder.