The one where the universe loses its 700th wonder...
"What is it?" asks Bellal, quite rightly. "It's another test, I think," replies the Doctor, before he starts to scan the "ornamental floor" with his sonic screwdriver. Of course, it's a "deadly floor", capable of pumping 7,000 volts into anybody who crosses it, but luckily the Doctor works out that it can be crossed safely by playing Venusian hopscotch (but regular hopscotch will do!). If only we'd had an inkling of what this danger was at the end of last week's episode, we might have felt more bothered. It was done so much better when script editor Terrance Dicks pinched this idea for The Five Doctors nine years later...
The city is basically made up of a series of Escape Rooms, which the Doctor and Bellal have to solve and survive as they move closer to the heart/ brain of the structure. The next room pits Bellal against the Doctor in an attempt to turn them against one another, but thankfully the Doctor overcomes the city's control over his Exxilon pal, again by using his sonic screwdriver. When the Doctor asks Bellal if he's ready to go on, the little fella says: "No. But we must?", and my heart weeps a tiny bit.