Not very deep however and "clear your mind" does seem like one of those often-used Moffat gimmicks. Feel could have been longer. But nice to have the occasional fun standalone episode which isn't too intense. And having Abslom Daak the Dalek Killer from Doctor Who Magazine appear was a moment that delighted this Expanded Universe fan. So is the 26th century chainsword-wielding deconstructive 90s anti-hero now canon in the Whoniverse? This episode had a shapeshifter. Will a shapeshifting penguin be next?
Overall not that profound but not meant to be and enjoyable all the same.