When a transporter malfunction strands an away team on a freezing planet, it also creates evil duplicates of Captain Kirk and a weird dog. But after naughty Kirk takes things too far with Yeoman Rand, nice Kirk is convinced to face the fear of recombination. Who is allowed to do father-in-law impressions? What’s the best way to heal an open wound? When would Spock beat Kirk at 3D chess? It’s the episode that’s at the headwaters of a trope.