The Karadax were the species native to the planet of the same name. They looked like walking sharks. They were always being defeated by the Tenth Doctor, so they calculated that removing Martha would make him unable to defeat them. They stole an android from the Museum of the Doctor, reprogrammed it to look like an older Martha, and sent it back in time to convince her to stop travelling with the Doctor. The real Martha soon figured out the "older Martha" was a duplicate, but a Karadax came back in person. The android had the thoughts and feelings of Martha, so she shocked the Karadax, sending him back to his time. After this, the Doctor decided to check up on the Karadax every few hundred years. (PROSE: Rewriting History)