"Mirror, Mirror" (TOS-R), again.
My disk containing "City on the Edge of Forever" and "Tomorrow is Yesterday" has a big scratch, making them unviewable in entirety.
Look up using toothpaste to fix a scratched DVD. It'll work long enough for you to rip the files from the DVD to your PC, so you can at least view them that way.
I saved my copy of Madden NFL 12 for the XBOX 360 using toothpaste.