Syntax:
set contour { base | surface | both }
set nocontour
If no option is provided to set contour, the default is base. The three options specify where to draw the contours: base draws the contours on the grid base where the x/ytics are placed, surface draws the contours on the surfaces themselves, and both draws the contours on both the base and the surface.
See also set cntrparam for the parameters that affect the drawing of contours.