Overlaying contour lines for a specific value

Hi! I created a scatter plot representing a ‘matrix’ of values and coloured it in using heatmap. I want to overlay a contour line on it for a specific value in this matrix but I can’t figure out how – any help would be greatly appreciated! This is the code that produces my plot:

plot = scatter(x, y, matrix, st=:heatmap, xlabel="x vals ()", ylabel="y vals ()", title="blah blah")