Display a slice of a 3D Image

Hello, I am new at OpenGL, WebGL programming.

This is the problem I have at hand:

1.I have a cube where every pixel has a color attribute.
2.I want to take a slice and display it
3.The slice is given by an arbitrary plane

I am a bit lost because so far I have been working with polygons, normals, colors, lights, etc. This is a different cat to skin though.

I would appreciate any ideas, I am pretty sure this has been solved for OpenGL but I am aware of the fact that WebGL has some limitations (no glDrawPixels for instance), so I am not sure of how this can be done there.

Thank you,