Preheat the oven to 180°C/350°F/Gas Mark four.
Grease a 20 cm (eight-inch) pie dish and line with baking parchment.
Roll out two-thirds of the pastry on a lightly floured surface until it is large enough to fit over the top of your pie dish with some overhang.
Transfer to the dish and press into the base and up the sides. Trim off any excess pastry with a sharp knife.
Refrigerate for 30 minutes.
In a bowl, mix together the sugar, orange zest, and cinnamon until well combined.
Add the plums and flour, tossing everything together until coated. Stir in a few tbsp of juice from the bowl.
Spoon into your pastry case and top with overhanging pastry, crimping the edges together to seal.
Cut a small hole or slit in middle of pie crust to allow steam to escape then transfer to oven and bake for 40-45 minutes until golden brown.
Serve warm with custard, cream, or ice cream.