The Curse of Peladon
In the serial , the alien time traveller the Third Doctor ( Jon Pertwee ) and his travelling companion Jo Grant ( Katy Manning ) discover the High Priest Hepesh ( Geoffrey Toone ) conspiring to stop Peladon from joining the Galactic Federation so that the old ways on the planet are preserved . 
<triplet> Third Doctor <subj> Jon Pertwee <obj> performer <triplet> Jo Grant <subj> Third Doctor <obj> sidekick of <subj> Katy Manning <obj> performer