As I remember the story was that the Doctor and Martha arrive in an underwater city and trip over a dead body - leading them to discover that the underwater setting is an illusion covering up the fact that the rest of the planet has been destroyed by nuclear war. The dead guy had been murdered because he'd found out the truth. The big bad is eventually shot by the dead man's son, who vows to tell the people the truth, no matter the cost to their peace of mind and way of life. It's from Panini's 2008 Doctor Who Storybook