In this episode Simon and Lewis are accompanied by their old friend Duncan as they try out the Hack/Mine mod. The mod itself adds a lot of content to the game, from the ability to choose a roleplaying class to brand-new areas.
The descrition reads: Lewis and Simon are joined by Duncan (http://www.youtube.com/Yogscastlalna) as they play the most dungeon crawlinest, hackin' and slashinest mod for Minecraft this here side of the Rio! With three playable classes with unique abilities, new area systems, thousands of different magical items and randomly generated dungeons! Hack, Slash, Mine!
The mod is available for download here: https://yogscast.com/showthread.php?46534-MOD-Hack-Slash-Mine!-(Magical-Loot-System-Randomized-Dungeons-and-More!!! (Note: The mod is still under development, as of April 2012, and may be a bit buggy.)