If you’re going to write a time travel story, you either totally ignore all the possible, unintentional ramifications of transtemporal travel, or you make the story about those ramifications. Either way, doing so requires a fair amount of narrative skill. That is, at least make it entertaining enough so that hidebound literalists and beady-eyed nitpickers don’t feel compelled to tear it apart. Gosh, this arc is infuriating. Given his seemingly supernatural gifts, surely there was some way that Hedley could have gotten back the dreary magic helmet. He’s had over 40 years to do it! But noooooo, he was content to leave it in Donna’s possession, and now it’s disappeared (and how does he even know this?). As a result, he’s “stranded” in space and time, and, nothing against janitors, but it’s probably a pretty mundane existence for someone capable of time travel and mind control. But hey, at least the music’s good!