In 1966, the unprecedented occurred: the popular William Hartnell left Doctor Who, and a man named Patrick Troughton took over the role. He was absolutely nothing like the crotchety old Doctor viewers knew well -- this new incarnation was feisty, mysterious and occasionally prone to fits of hysteria. The series was changing, and the viewers were along for a wild ride. Troughton was immediately a new type of Doctor -- much more hands-on, more physical, and more prone to slapstick. To that end, in his second adventure, he gained the services of Jamie MacCrimmon (Frazer Hines), a young Scottish soldier who would see him through to the bitter end, joining the travels with Ben and Polly (Michael Craze and Anneke Wills), who witnessed the transformation. Along the way, Ben and Polly would depart his company, only to be replaced by two very independent young ladies from different backgrounds: 19th century socialite Victoria Waterfield (Deborah Watling) and futuristic scientist Zoe Heriot (Wendy Padbury).

Along the way, the second Doctor faced terrifying monsters and alien adversaries. The Daleks and the Cybermen returned to haunt his travels, and we were introduced to new threats: the fiendish Great Intelligence and its horrific servants, the Yeti; the conniving Martian soldiers, the Ice Warriors; the savage Macra; and the cruel Dominators. The Doctor also spent a good deal of time on Earth, be it in the Himalayas of the early 20th century, the wiles of Victorian London, or the plains of Culloden on the Scottish moors. He would face a duplicate of himself bent on world domination, a mad scientist plotting to destroy ancient Atlantis, and a strange Land of Fiction which bent to the whims of its guests. But it was back in 1960's London that the Doctor joined forces with a man whose destiny would be forever intertwined with his own: Alastair Lethbridge-Stewart (Nicholas Courtney), soon to become a Brigadier of the newly created United Nations Intelligence Taskforce, or UNIT for short.

And at the end of his travels, viewers would get their first glimpse of the Doctor's mysterious origins and his complicated past, as he is forced to call upon his own people -- the Time Lords -- in a bid to save his friends and countless innocents caught up in a brutal alien war game. Put on trial for his actions, as depicted in the series' first six seasons, he would soon be exiled to Earth and regenerated into his successor, Jon Pertwee. Troughton would later return for several appearances alongside Pertwee, William Hartnell, Peter Davison and Colin Baker, before his tragic death at an overseas convention in early 1987.

Sadly, the Troughton era of Doctor Who is hugely notable for what does not exist as opposed to what does; only one story in Troughton's first two seasons exists in its entirety, ten stories only exist partially (most with one or two episodes out of 4 or 6), and four are lost in their entirety, including his first story, "The Power of the Daleks"; Jamie's first adventure, "The Highlanders"; and "Fury From the Deep," considered by many to be one of the greatest serials of the program in the 1960's. The search continues...