Doctor Who episode 730: The Age of Steel (20/5/2006)
‘Rose, I’m coming to get you!’ Mickey saves the day through dogged determination, computer skills and refusing to live down to the Doctor and Rose’s expectations. In a curious way, he’s grown more than Rose, from useless comedy sidekick to heartsick tin dog to action hero. No wonder he chooses to stay behind in a universe that shows some appreciation for him, where he’s able to make a difference as his own man, not the Doctor’s assistant. It’s pretty much the only time in the new series a companion makes that choice to grow beyond the Doctor (usually there’s some terrible reason why they can never see him again), and it works so effectively that RTD pretty much repeats the trick with Martha (let’s skip over that very unfortunate coincidence).