A clone of Martha Jones was created by the Sontarans' cloning pool to aid them in their invasion of Earth.
Biography
Martha's clone originated in the cloning pool located in the ATMOS Factory, and was originally a human blank slate until Martha's specific genetic material was transmitted to it. The clone acted as the Doctor's assistant and link with UNIT. The real Martha lay in a trance state linked to technology which transferred her memories to the clone, hypnotized UNIT troopers having lured her into the room at the factory. "Martha" stopped UNIT's missiles from destroying the Sontaran flagship in Earth orbit.
The Doctor had noticed many differences between the clone and the original (for one thing, according to him, she "smelled funny"), though he did not inform the copy that he had guessed until she turned a gun on him. When the Doctor disconnected the real Martha from the Sontaran technology, the clone began to die. The real Martha felt sorry for her and convinced her to tell them the nature of the unidentified component of ATMOS gas emissions, clone feed, before she died. Even disconnected from Martha, she still retained at least some of her memories and this was what caused her last act to be her helping the Doctor and Martha. Before she died, she told Martha that she (Martha) had so many plans for her life and she should go out and do them. (DW: The Sontaran Stratagem/The Poison Sky)