The first four episodes (out of six) are standard sfnal fare - indeed, perhaps Doctor Who's closest early approach to the monster-of-the-week concept (bottled brains, mutant plants, giant wolves) - but I think it lifts itself into something rather superior in the last two episodes, a courtroom drama where the Doctor is defending Ian against a charge of murder, with only circumstantial evidence against him, but collusion between key officials and the real culprits. (A helpful official asks Ian, "Who is he?" "Who?" replies Ian, rhetorically agreeing.) Though the trial judges do bear a disturbing resemblance to the High Priests in the 1973 film of Jesus Christ Superstar.
The climax really does take us back to the future, as the First Doctor, having assembled all of the Keys of Marinus, makes the same decision as the Fourth Doctor at the end of The Armageddon Factor; though with more explosive consequences. One of the heavily costumed bad guys nearly trips over his own shoes as he tries to bring our heroes to their doom, but recovers from it quickly; and that's part of the charm, really.
Is this final snippet of information for real?
Amongst later changes made to Nation's scripts was the removal of a TARDIS sequence from episode one, The Sea Of Death. Here it was revealed that the reason the Doctor and Susan had been on Earth in 1963 was because the Doctor had visited the British Broadcasting Corporation to get help repairing the colour scanner in the TARDIS, which was showing only monochrome images. He had been in such a bad mood upon his return to the TARDIS because the BBC had been “infernally secretive”!I have my doubts.