John Simm took on the role of the Master in 2007 and played him as a brilliant mad man.  His storyline begins with hints and references to the name Saxon and before we know it, he is Prime Minister of the United Kingdom, with a plan to wipe out the human race and the Doctor! His story arc concludes with the first ever dual Master episodes with He and Missy at first working together before betrayal ends the partnership. he was our master until 2010 but has appeared since then.