Wow. That episode...well, sucked. And sucked hard.
So, let me get this straight: the Celestial Toymaker, a being from outside our normal universe, an entity that can turn bullets into rose petals, was defeated because he...couldn't catch a ball? And, having not caught the ball, did not have the ability to make the ball simply fly back up into his hand? Or teleport to his hand? Or, since we saw several of him in rapid succession doing his "dance video" sequence there in UNIT headquarters, he couldn't suddenly appear on the ground outside UNIT headquarters and catch the ball after it fell over the edge of the projecting bit of building they were playing on? Any of those seem like they would be well within the "rules" of catch, which seem to be "catch the ball after it is thrown by another player," so it's not like he would be cheating by doing so.
And the Doctor, who earlier defeated the Celestial Toymaker at his own games in an earlier incarnation, now had no better plan to stop him than "let's cut cards and see who picks the higher card?" This is the Doctor's brilliant stratagem? (I guess it's a good thing he had "let's play catch" as a brilliant backup plan, huh?)
And didn't the Doctor and Donna have an issue, in the previous adventure (which was literally just minutes ago in their timeline), where separating was a bad idea? That lesson apparently didn't sink in, as the first thing they do when trapped inside the Toymaker's workshop halls of endless doors is to separate themselves on either side of a door, then each go wandering off. (Oh, but the door was locked when it was closed! Too bad the Doctor doesn't have a sonic screwdriver designed to open locked doors, huh?)
This was insulting on so many levels. The concept of "bi-regeneration" was terrible (as others have pointed out, it's basically telling Gatwa "Congratulations! You're the new Doctor! But we have a potentially better Doctor on hand to rescue the series in case you run it into the ground" - what a vote of confidence!), the plot was nonsensical (I'm with the new science adviser: in all these years, we've never been able to pick up the image of a laughing ventriloquist dummy hidden behind every single television broadcast, ever?), and even the ending, with the 14th Doctor "learning" what it's like to stay in one time period for awhile was ridiculous - didn't the 12th Doctor spend "one final night" with River Song on a planet where "one night" equals something like 61 years? And the TARDIS has an oversized wacky prop mallet stored in its floorplates for emergencies, in case you ever need to whack another TARDIS out of the existing one?
I can only shake my head in disbelief. Great job, Davies.
I wish Gatwa well in the role. Hopefully he'll be given better scripts to work with in the rest of his tenure as the Doctor.
Johnathan