I must be stupid because I've seen this a few times and still haven't figured it out. Can someone tell me how Trish, who confronts Hugh after the murder telling him that she guesses he did it, provide him with an alibi by being in disguise, something that can only have been contrived before the murder??

This fact is certainly an odd part of the story. You're correct though. Anyone would have thought that something strange was happening when one's boss asks you to wear a mask of his face, and speed in his car past a police speed-enforcement camera. Should she have figured out that he was setting up an alibi for himself - for SOME reason - ? You would think so! Or maybe Hugh just wanted to get caught!

I had not seen this episode since it originally aired until last night. I must admit I was thoroughly confused when Trish confronts Hugh and tells him she believes he murdered Marcy. Trish mentions lending Hugh her car but says nothing about her wearing a mask of Hugh's face and that he had her speed through a site with a camera that would capture the image and send out a citation for speeding. Now, if a business partner of mine asked me to don such a thing while I drove around in his car, I would find it pretty curious. First of all, how was Trish supposed to see through the cardboard mask? If holes were cut out, that fact would have been apparent when the images were blown up. I watched these sections several times and was surprised that nothing was said to explain these big plot holes.

When the speeding ticket is revealed we are forced to think that Trish was in on the scheme all along, because how could she not see that she was being used as a phony alibi for Hugh.

Still, an enjoyable episode. I was glad to see Dabney Coleman as Hugh, caught in his lies.