The First Elder was the leader of the Sensorites in the 28th century, and was identified by his wearing two crossing black sashes. He was on the Sense Sphere when an Earth ship with a crew of five humans landed on his world — and, indeed, when the First Doctor, Ian Chesterton, Barbara Wright and Susan Foreman arrived on a human ship trapped in orbit via TARDIS.
Following an unfortunate initial contact with humans who, because they had been driven mad by Sensorite telepathy, had poisoned the Sense Sphere's aqueducts with atropine and blew up deserters, the First Elder forbade further contact with ships from Earth. After this ban had been in place for years, another ship, commanded by Captain Maitland, approached the Sense Sphere. The First Elder ordered the humans be kept prisoners on their ship, which was forced to remain in orbit.
When the Doctor's TARDIS landed on the Earth ship, the Sensorites detected that Susan was telepathic. The First Elder requested she be brought to the surface, but the Doctor refused to allow his granddaughter to go unless accompanied by Ian and himself. When they arrived on the Sense Sphere, the First Elder and the Doctor came to an agreement to cure what they thought was a disease.
After Ian became ill from drinking water from the aqueducts, the Doctor surmised that the water was poisoned and this "disease" did not come from the destruction of the humans' ship. On consulting with his suspicious Second Elder, the First Elder chose not to give the Doctor access to his TARDIS, but agreed to allow him access to a lab on the Sense Sphere. The Doctor found a cure for the poison.
The First Elder began to trust the Doctor after he cured the Sensorites, but the City Administrator did not share this trust. He murdered the Second Elder and unsuccessfully framed the Doctor. Following the Doctor's advice, the City Administrator was promoted to Second Elder. After the three humans were found by the Doctor in the aqueducts, the First Elder allowed Captain Maitland to return to Earth with them. He also found out that his new Second Elder was a traitor, and had him banished for life to the outer wastes of the Sense Sphere. (TV: The Sensorites)