This is one of the most fun episodes since 2005. The flatland creatures are the type of hard-sci-fi/heavy maths concept which would make Christopher Bidmead drool, and trapping the Doctor in the TARDIS is a fun way to pull off a "Doctor Lite" episode which doesn't seem like one (I'm guessing since all his scenes were in the control room, they could get a bit more out of Capaldi than many "Doctor Lite" stories. Boy, that Danny Pink/Clara/Doctor relationship is turning toxic, isn't it?