From TARDIS Index File, the free Doctor Who reference.

For other characters with the same name, see John.

John was a mineralogist on Captain Maitland's ship. During the expedition to the Sense Sphere, he discovered that the planet was rich in molybdenum, forcing the Sensorites to attack his mind. He, along with the rest of the crew, was held prisoner on their ship. His mind was eventually restored, and he was allowed to return to Earth after promising to keep the Sensorites' existence secret. (DW: The Sensorites)