The one where a Dalek chases the Doctor up a ventilation shaft...
The Daleks may not be able to construct their own hot air balloon, but they can follow our floating Thal pals up the ventilation shaft on one of their anti-gravitational discs! How cool is that? The disc itself is literally a black disc, and nothing like the hoverbouts which have populated Doctor Who spin-off fiction since they first appeared in the Cadet cigarette sweets stories of 1964. In fact, it's a real shame the hoverbouts (sometimes called transolar discs) never appeared in the classic TV series properly, only ever in comics, novels and audio stories.
In a nice nod to the scene in The Ambush where the Doctor and friends bung a heavy object down the lift shaft to stop the approaching Dalek, the Doctor and the Thals chuck a load of rocks down the ventilation shaft and send the anti-gravitational Dalek crashing back down to the bottom. All very satisfying!