Preheat oven to 375 degrees.
Mix egg, sugar, flour, and water and let stand while getting the rhubarb and crust ready.
Place uncooked crust into pie plate.
Pour in cut up rhubarb.
Pour filling mixture over rhubarb.
Dot with butter.
Bake at 375 degrees for 45 minutes.
For best results, cover the rim of the pie with foil until the last few minutes of baking.