Doctor Who: Kingdom of Lies

On the planet Cicero Prime, the kingdom of Cardenas is divided, with the whole population forced to swear allegiance to either the effete Duke or the fiery, hard-edged Duchess. This is a situation both parties have grown tired of. What use is half a kingdom when, thanks to a carefully engineered murder, you could have it all? Surely, neither of them would be rash enough to summon the deadly off-world assassin The Scorpion to help with their problem? And surely, this terrifying figure wouldn't arrive wearing a long cream coat and striped trousers...?