Straighten an edge loop on a slopped surface

How can I straighten the vertices in the edge loop on this slope:


I’ve tried edge and vertex slide which wouldn’t work and also translate/move along a constrained axis. Still no joy.

Assuming that slope is planar

Select your edge loop and change the Transform Orientation to Normal instead of the default Global.
http://i.imgur.com/Sg66paA.jpg

http://i.imgur.com/GNXDbQE.jpg

Notice the 3D manipulator change, showing on my screenshot that on my mesh the transform orientation i’ll need to strengthen on is the normal Y axis

So i press S then Y Y ( i need to press Y 2 times so Blender understand i’m using the normal Y axis, not the global one) and press 0
result :
http://i.imgur.com/pqm8YjB.jpg