The one where the Doctor confronts Ice Lord Slaar...
The ubiquitous use of the BBC foam machine in the Troughton era really reached its zenith in this episode, having been used to perhaps its advisable limit in Fury from the Deep. Here, the foam loses all sense of threat as a deadly alien fungus and simply becomes a giant playground for Patrick Troughton to play in. It's all very silly and drawn out (parts of the reprise are still being shown three minutes in!), and forms part of a greater atmosphere of slapstick in this episode's opening minutes.
The acres of foam aside, Troughton resorts to gurning, oohing and aahing, and generally fooling around in a misplaced attempt to sell the jeopardy, while inside the weather control bureau, Frazer Hines is embroiled in a Benny Hill-style runaround playing peekaboo with an Ice Warrior, and Wendy Padbury is sliding around the corridors in an endless sprint for the front door. By the time Zoe opens the door and the foam floods into the bureau, the Doctor with it, Padbury has already succumbed to the silliness of it all and is clearly seen laughing her head off!