Doctor Who: 10 Storylines That Will Never Be Revisited
8. Adam And His Alien Forehead
Adam was a short-lived companion of the Ninth Doctor, first appearing in the brilliant Dalek as a computer whiz who ended up battling the titular villain with the Doctor and Rose. He then joined them for one more trip, before vanishing from the show entirely.
However, he wasn't killed, erased from time, or anything of the sort: he was left behind by the Doctor. After Adam installed some alien tech inside his own head, he almost gave the sinister Jagrafess control of the TARDIS, which, naturally, didn't sit well with the Doctor, who angrily left Adam at home.
And this is the last we saw of him on the TV screen. Under ordinary circumstances, his lack of additional appearances wouldn't really be noteworthy, but at the end of his last episode, Adam's forehead-hole is accidentally discovered by his mother, since it's revealed with a simple click of the fingers.
The point is, there's no way that Adam could live a normal life with this alien tech inside him, and the fact that the Doctor just left this potential danger on Earth is actually quite weird. Adam might want revenge against the Doctor for leaving him behind, and this tech might even give him the means to do it. But, strangely, we never heard a peep from him again.
He did pop up in some comics (where he actually got a redemption arc), but like we mentioned in our introduction, these have not been labelled as canon by the BBC.