The Invaders was a well-crafted Sci-Fi/Adventure series created by Quinn Martin that sadly only lasted 2 seasons on ABC.
Roy Thinnes plays the role of David Vincent, an architect who witnessed the landing of a spaceship in a remote desert in the Southwestern U.S. & finds out that the aliens who arrived have plans to take over the Earth. Throughout the series Vincent tries to convince anyone & everyone about the evil humanoid aliens, but with little success. However, the aliens' human guise is far from perfect since they have an extra finger, no pulse or heartbeat & most of all, when an alien is weakening or dying, they give off a reddish glow. They even dissolve into vapor when they die. Later in the 1st season, Vincent finds an ally in Edgar Scoville, the head of an electronics company along with 7 others who also discovered the evil aliens' plot as well.
I was very fortunate to have seen a couple of episodes on Chicago TV many years ago. I just hope that if i can't see this lost classic on TV, then maybe i can find it on DVD.