The Commander was the leader of an exploration mission from the Earth Empire.
The Commander made contact with the Sensorites, discovered that the Sense Sphere was rich in molybdenum and attempted to exploit its mineral wealth. Shortly afterwards, two of his crew deserted them in the spaceship, which the Commander destroyed. The surviving three crewmembers hid in the caves outside the Sensorites' city. They began adding atropine to their water supply in the hopes of killing them all and claiming the planet for themselves. Their scheme was discovered by the First Doctor and they were taken into custody by Maitland's crew. (TV: "A Desperate Venture")