...regularly featuring violence on par with the R-rated horror movies of the time. Gore was commonplace in the first season (dead aliens and their tossed away hosts' bodies melted in a grotesque puddle), and the super-strong Mor-Taxans had no compunctions about mutilating any poor schlep who got in their way. One of their trademark methods of murder would be gouged out eyes courtesy of the third arm that would often burst out from their chest.

During the first season, the aliens were lead by a triad known as the Advocacy. They were a part of their society's ruling class, overseeing the invasion force on earth while their leaders, the invisible and never heard Council, remained back on Mor-Tax. Outfitted throughout most of the season in contamination suits that pumped coolant to counteract the killing heat of the radiation they needed, they stayed in their base of operation: a cavern in the Nevada desert, which was perfect due to the ambient radiation from the atomic bomb tests. They rarely went into battle because without them, the lower classes would have no guidance and be useless.

Their goal is to pick up where they left off in 1953, by wiping out humanity. For the aliens' hatred of human beings goes beyond simple prejudice. Having come from a planet that can be compared to the Garden of Eden based on description, the aliens see that humans do nothing but snuff it out. Without them, they can help bring out the vegetation, and better replicate the conditions of their world. To accomplish this, they seek out weapons (some of which is their own left behind), help amass their army, engage in filtration, and all sorts of acts of warfare. But to make things more problematic, they also must find immunity against the germs that befell them in '53.

If the war between humanity and the aliens wasn't enough, the season also dropped other anomalies into the mix:

Quinn - an alien trapped in a human host since the invasion of '53, mysteriously immune to bacteria, and ready to play both of the major warring factions against each other for his own favour. The Qar'To - an unknown alien race represented by a synth sent to Earth, having deadly reasons for wanting the aliens dead and humanity preserved.

Project 9 - a government organization much like the Blackwood Project, but they have a shadowy hand in alien research.
Also inserted for the Blackwood Team's side was Sylvia Van Buren, a colleague of Dr. Forrester in the years following the war who had developed a strange ability to sense the aliens and even often make predictions with a fairly accurate rate. And while the aliens had science on their sides, the supernatural elements wielded by Joseph Lonetree (whose presence was seemingly foreshadowed in the first episode) seems to be something that the aliens cannot defeat. The team even made friends with the remaining Grover's Mill militia of 1938 who had their own run in with the aliens, not once but twice.

A recurring element with the aliens was the number 3. An extention from the film, when it came to the aliens, just about everything had some root in the number three - from their caste system (Ruling Class, Soldiers, and Scientists) to their body (three arms) to their planet (their planet is the third from the sun), weaponary (in "The Resurrection", they make bolas with three weighted ends), and even their mating ritual was every nine years. The appearance of the number in some form (sometimes obvious, sometimes subtle) is sprinkled throughout the season in reference to the aliens.

The episodes all had (often ironic) Biblical titles, such "The Walls of Jericho", "To Heal the Leper", and "Among the Philistines".

"To Life Immortal" ("do na decatae" as it would be said phonically), a phrase by which the aliens seemed to sum up their belief system, became a hallmark of the series as it was exchanged between aliens or cried before one would take its own life in honour, and became a well-deserved catchphrase for fans.

FROM : WIKIPEDIA