In this episode Martha considers leaving the Doctor and their adventures. The Doctor and Martha must deal with man named Lazarus who claims that the changes he'll bring will aid in the progress of the human race. But the Doctor knows that there's more than meets the eye with Lazarus and his device....This episode is one of many highlights during Tennant's years as the Doctor. David's acting in this episode is one of many reasons why the Tenth Doctor is well-known and so well-loved among Doctor Who fans.