Who Magazine series overview, in issue 407 (pp26-29). The Discontinuity Guide , which counts the unbroadcast serial Shada , lists this as story number 135. Region 1 DVD releases follow The Discontinuity Guide numbering system. ^ http://shillpages.com/dw/other.htm ^ Shaun Lyon et al. (2007-03-31). "Planet of Fire" . Outpost Gallifrey . http://gallifreyone.com/episode.php?id=6q .