diff --git a/lib/matplotlib/sphinxext/plot_directive.py b/lib/matplotlib/sphinxext/plot_directive.py
index 1e92f5a29814..795c39daac73 100644
--- a/lib/matplotlib/sphinxext/plot_directive.py
+++ b/lib/matplotlib/sphinxext/plot_directive.py
@@ -90,7 +90,11 @@
         Whether to show a link to the source in HTML.
 
     plot_pre_code
-        Code that should be executed before each plot.
+        Code that should be executed before each plot. If not specified or None
+        it will default to a string containing::
+
+            import numpy as np
+            from matplotlib import pyplot as plt
 
     plot_basedir
         Base directory, to which ``plot::`` file names are relative