"Penniless recently-orphaned Anne Beddingfeld witnesses a man falling to his death in a crowded London tube station. A mysterious, brown-suited man claiming to be a doctor examines the body, but drops a piece of paper that Anne picks up, with a number and "Kilmorden Castle" written on it. A famous dancer is then reported murdered, with links to the tube station victim, and Anne begins to investigate. She discovers that Kilmorden Castle is the name of a cruise boat, and backed by a newspaper magnate, sails on it to South Africa to follow the trail."
This is the descrition for an Agatha Christie novel from 1924. The title of that novel:
Now who else do we know who wears a brown suit and claims to be a doctor...hmm..
We know that Agatha Christie is appearing in season 4 of Doctor Who. Are they doing a quasi adaptation?
Hattip to Millarworld for more info look to Wikipedia