I would use contour with specification of level and combine with hold on.
Take al look to the documentation
contour(Z,v) draws a contour plot of matrix Z with contour lines at the data values specified in the monotonically increasing vector v. The number of contour levels is equal to length(v). To draw a single contour of level i, use contour(Z,[i i]). Specifying the vector v sets the LevelListMode to manual to allow user control over contour levels.
