I have been trying to work out why the idea of John Simm as the Master in Doctor Who is so compelling. By my calculation, Simm is the eighth actor to play the Doctor's nemesis, who originally returned to the revived series in the form of Derek Jacobi. Of course, there is innate (not to say topical) appeal in the storyline that concludes in tonight's season finale: the diabolical Timelord, masquerading as populist Prime Minister Harold Saxon, taking control of the public by manipulating the mobile phone network. But there's something special about Simm, that was sealed by his performance in the magnificent retro cop drama, Life on Mars. Over the past few years - in the movie Human Traffic, and then television roles in The Lakes and State of Play - he has somehow become the closest we have to a screen incarnation of modern Britain. Michael Sheen is the great actor-as-mimic of our times (Blair, Frost, Kenneth Williams, and soon Brian Clough). But Simm somehow holds up a mirror to the culture, in the same way that Jimmy Stewart did for post-war America and Michael Douglas did in the Eighties. Simm would have been an obvious choice to play the Doctor. But it is much more unnerving and clever to have him as the villain, exploiting his onscreen familiarity to unsettle the viewer, just as Stewart did in Vertigo. I shall be watching tonight.