The Magician's OathSynopsis :
"You must never tell. Not a soul. That's the magician's oath."
A heatwave in July and a tube train is discovered buried in twenty inches of snow. A Saturday afternoon in Hyde Park and scores of people are instantly frozen to death where they stand while the sun beats down from the sky. Freak weather conditions in London, and the Third Doctor and UNIT are called in to find the cause.
Meanwhile, a street magician, who was witnessed at the scene of the tragedy, entertains crowds in Covent Garden. As Jo Grant and Mike Yates disobey orders and investigate alone, they discover an enemy wth terrifying powers. And they may not live to share his secrets... Casting :
Mike Yates - Richard Franklin
Diamond Jack - Michael Chance
Source : http://tardis.wikia.com/wiki/The_Magician%27s_Oath_%28audio_story%29