"The Invaders" was a 1960s tv show that starred Roy Thinnes and had to do with aliens that invaded the Earth but that were hidden amongst us. Roy's character was one of the few that understood what was going on and waged a one man crusade against it.
I was not alive during its original run. I did watch it as a block of Sunday afternoon programming that the local station Channel 68 had. The block included "Voyage to the Bottom of the Sea," "The Rookies" and "Charlie's Angels." I thought the pairing of the last two were interesting since "The Rookies" was Kate Jackson's first tv series and "Charlie's Angels" was the series that followed.
At any rate, "The Invaders" has a fan site: http://www.theinvaders.co.uk/.