Doctor Who, the legendary British sci-fi series, turns 50 this year, but the BBC never expected it to last so long, and foolishly destroyed or recorded over many early episodes. Fortunately, two lost stories, “The Enemy of the World” and “The Web of Fear,” were found in Nigeria, remastered, and have been released exclusively on iTunes — to be seen for the first time in over 40 years. The two stories are available for $9.99 apiece and feature Patrick Troughton as the Second Doctor and feature Nicholas Courtney’s first appearance as the beloved Colonel (later Brigadier) Lethbridge-Stewart. Unfortunately, episode 3 of “Web of Fear” is still missing, though the serial features a reconstructed episode with audio and still images. There are still 27 Doctor Who stories that are partially or entirely missing.