As we all know, Freema Agyeman played someone who suffered at the hands of the Cybermen at the end of Series 2, but this time, she is Martha Jones. How do they battle the continuity issues? By making Martha Jones the cousin of the person who died! This is from the BBC press office.

Quote:
This time she plays tough, independent Martha Jones, who is revealed to be a cousin of her earlier character.


http://news.bbc.co.uk/1/hi/entertainment/6497163.stm