Question: intersection of a plane and a matrixplot surface

How would one plot the intersection of the surface z=3.5 and the surface described by a matrixplot

Here is an arbitrary matrixplot surface for an example.

matrixplot([[3, 4, 5, 1, 1, 1, 2, 3, 2, 1, 7, 8], [2, 3, 4, 1, 2, 1, 2, 1, 2, 1, 2, 5], [7, 8, 7, 6, 7, 8, 4, 5, 3, 2, 1, 2], [2, 1, 2, 4, 5, 6, 5, 6, 7, 6, 5, 6], [2, 3, 2, 4, 5, 4, 5, 6, 7, 6, 5, 4], [9, 8, 7, 8, 9, 8, 9, 8, 7, 8, 9, 8], [9, 8, 7, 8, 8, 8, 7, 8, 6, 8, 7, 6]]);

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