"Earthbound" is one of Space: 1999's finest hours.
When Space premiered back in September of 1975, I was there in front of the colour tube to welcome another starfield patch... even if stars were a bit on the scarce side in this colourful import from the UK. Since I know the old SF television series very well, due to my then space cadet rating, I can pick and choose what I want to watch. And I choose this episode.
Simmonds is the floating variable in "Earthbound". Visiting alien leader Zantor, portrayed most effectively by some dude named Christopher Lee, is an unknown quantity in a friend-or-foe sense; but having the boisterous bureaucrat producing his own sneaky threat made for interpersonal drama that was all too rare on Space: 1999. (Year One, that is; Year Two was a huge improvement in that regard.) This dynamic sets up and plays out the themes of "nobility" and "trust".