Preheat the oven to 350°F.
In a large bowl, combine the molasses, boiling water, and baking soda. Pour the mixture into the unbaked pie shell.
To make the topping: In a large bowl, mix the flour with the sugar, shortening, and salt. Spread this on top of the molasses mixture in the pie shell.
Bake until the center of the pie is set, about 45 minutes.
Cool on a wire rack or windowsill until the pie is firm, about 45 minutes.
Store any leftovers in a sealed cake safe. I've been told pie will keep for about 5 days, but the pie is gone before then.