Move rack to lower third of oven. Preheat oven to 400°F. Make filling: Mix plums, sugar, tapioca and lemon juice well
Roll out 1 disc of dough into an 11-to-12-inch round and fit gently into a 9-inch pie pan. Crimp edge.
Spoon filling into pie shell. Dot with butter; set aside.
Roll out remaining piece of dough into a 12-inch round. Cut into 12 strips, each 3/4 inch wide. Lay 6 dough strips evenly over pie, using longer strips in center and shorter ones on sides. Fold back every other strip to its midway point. Place a new strip at a 90-degree angle over unfolded strips. Unfold folded strips, crossing new one. Then fold back alternate strips and place another new strip parallel to first one. Continue weaving in this fashion so that you have 3 strips on this half of pie. Then weave crust on other half of pie in same fashion.
Place pie on baking sheet and bake until filling is bubbly and crust is golden, about 1 hour. Transfer to a rack to cool for about 1 hour before serving.