A good first episode in which we get a good handle on all the elements of the show. The aliens are simply portrayed as humanoid but look weird with their shiny green hair and skin which we learn is only that colour because of the green oxygenated liquid they breathe whilst in flight. The UFO design is simplistic but is very effective due to the rotating outer shell of the craft.
We see all the hardware that SHADO have, submarines, Moonbase interceptors and the Moonbase set is very colourful and look real.
Ed Bishop as Straker comes across as quite a solid leader, a bit unemotional but not cold.
George Sewell's Freeman providing some lighter relief. The plot about aliens coming to Earth to harvest human organs is chilling but the revelation that they're sterile and a dying race that age rapidly in Earth atmosphere presents them as desperate to come here knowing the risks. As Straker says they are driven by the powerful force of survival. Particular mention for Gabrielle Drake as Moonbase leader Gay Ellis, who looks great as do all the girls on the moon. Apparently the purple wigs were Sylvia Anderson's design and meant to be just part of their uniforms with no special relevance. Dancer Peter Gordeno appears in this episode as does children's TV presenter Ayshea Brough.