Commentators have offered a range of explanations as to the exact nature of the Mysterons. According to Nicholas J. Cull, they are a species of "invisible" alien, while Alasdair Wilkins and Sophie Bushwick of the website io9 describe them as "an unfathomable race of probably non-corporeal entities". The tie-in publication "The Complete Book of Captain Scarlet" by Chris Bentley refutes the notion that the Mysterons of the original series are a type of energy being, instead referring to them as "sentient computers" forming a group mind. The computer complex on Mars is the legacy of the original, corporeal Mysteron race – described as being of extragalactic origin and "masters in the art of computer technology" – who colonised the planet in the 2nd millennium BC and abandoned it at the start of the 20th century AD.