"When did you start believing in impossible heroes?" "Don't you know?"
IN THIS ONE... The Doctor finds it hard to accept Robin Hood as real.
REVIEW: If I know Mark Gatiss, he's inserted all kinds of sly references to Robin Hoods of film and television past (like the pic of Troughton as the Prince of Thieves, cheeky) into his romp, I'm just not very good at spotting them for lack of general Robin knowledge. His references to past Who, however, are more noticeable to me - Venusian Aikido (Hai!), Enlightenment, the arrow in the TARDIS (neat healing ability), the feel of both The Time Warrior and The Androids of Tara, the Mini-scope reference, Marion left the way K9 was in School Reunion (this was a touch confusing; was she in the TARDIS or just standing behind the box all that time?), and it's probably accidental, but holy crap does Ben Miller (as the Sheriff of Nottingham) look like the Anthony Ainsley Master! But in reference both heroes (Doctor/Robin), Gatiss also mocks and inverts their tropes. This is why Clara is identified as the leader of the band (part of a over-arcing theme), and rightly so, since her role and the Doctor's are inverted. Not only did she choose the destination, but she's the one showing him impossible and wondrous things while he acts as the straight man and like the audience goes "this isn't possible". Simply put, he keeps refusing to believe Robin Hood is real while also accomplishing feats just as unreasonable as the legend's - he may cheat the arrow tricks, but the spoon fight is quite real. As it turns out, so is Robin, so are the Merry Men, and so is the Sheriff. Sorry Doctor.
Unfortunately, Gatiss may have over-egged the romp. The entire situation, the fact the Doctor keeps trying to debunk it, and his childish competition with Robin (refreshingly non-romantic) should have been enough for the comedy to work. The robots and their castle spaceship introduce "comedy science" that has its own logic, which might as well be magic. The robots are easily defeated by a collection of reflective plates (there sure is a lot of weaponized dinnerware in this one), and the climax hinges on the ship drawing power from gold, its mere contact enough for it to work. Looks like they didn't even need to melt it down, they only needed to carry it in the hold. Throw in a reference to the Promised Land that doesn't jibe with the eventual revelation of what that is (but see Theories), and you have a glib mess of a plot on your hands. When anything can happen - down to the sonic screwdriver exploding hay - none of it feels particular real or gripping.
While I could jettison that plot, when it comes to the characters, the concept of reality ties into the Doctor's exploration of his identity which is at the heart of Series 8. Robin tells the Doctor he's as real as real as the Time Lord is, a bit of ham-fisted meta-commentary, but the point is made. Robin and the Doctor are cut from the same cloth (which is what draws Clara to both, though again, not romantically), both are impossible heroes and up to a point, playing a role. Robin laughs too much, but the Doctor grumbles too much. Both are fronts, masks, to hide their true feelings. When they look at each other, they see their true selves - Robin is in reality sad, and the Doctor can't allow himself to show Clara just how much he enjoys himself - and it irritates them to see exactly what they're trying to hide. This is what their competition is about. Not the girl's attention (see 9, 10 and 11's interactions with Captain Jack, Rory, etc., even each other), but perhaps her approbation. She's the mum, or the big sister, not the love interest, and they act like brothers of a sort. The Doctor tries to humiliate Robin at every turn, and regardless of that fact it often bounces back on him, he puffs out his chest when his "baby brother" uses the same trick on the Sheriff he did at the river, with pride. If The Thick of It proves anything, it's that he's great at angry comedy (freeze-frame the scene where he puts on gloves for an echo of that Malcolm Tucker's rudeness), and Robot of Sherwood certainly gives him the opportunity to show that off.
THEORIES: If the Promised Land is Heaven/the Matrix, why are the Sherwood robots trying to reach it, or even know about it? It looks like a planet on the computer screen, which will turn out to be a red herring. The crucial clue is that their databanks also contain information on Robin Hood as a legend (and television character). In other words, the information is from the future, and maybe they are too. From there, it's not too much of a reach to assume Missy is setting traps for the Doctor all through history, either allying with various threats (the clockwork men and these guys, for example) or manipulating them with the promise of an afterlife that would other be denied mechanical men (non-sentients who surely are soulless). And there's another, much more important question: Where does Clara get such fabulous hair extensions? Mummy on the Orient Express will do the reverse, with a much shorter hairstyle. Can the TARDIS localize time to make your hair grow faster (and would it then gray up faster as well, over time?), or - probably the simpler explanation - does its wardrobe have future-tech wigs that look and act like real hair, won't fall off, etc. to allow travelers to better fit into other time and places?
REWATCHABILITY: Medium - An amusing trifle with make-it-up-as-you-go logic. Capaldi's comedy chops and the general charm of the cast save it from being rather stupid.