The Mysterons are a race of Martians who are never seen - they appear only as disembodied voices and the famous 'Eyes'. They can take over people and things, replacing them with a perfect replica under their control.
During a mission to Mars, Captain Black destroyed their base thinking them a threat. They immediately started hostilities, declaring a 'War of Nerves' on Earth and all earthlings. Among their fiendish plots they have turned Captain Brown into a human bomb, sabotaged Air Conditioning systems and plotted the death of Fashion Designers.
Ironically, their main earth agent is Captain Black - who was killed, revived under the control of the Mysterons, and now acts a spy and saboteur...