4Z2 - The Invasion of Time 2

It's difficult to tell where the line between The Doctor playing strange due to his bargain with the aliens ends, and the growing onscreen madness of Tom Baker begins. It's an oft-repeated tale that Baker had the idea that his next companion should be a cabbage perched upon his shoulder who he could talk to and dispense the plot to. It seems as if one scene in Borusa's office was Baker's test run with an invisible cabbage.

The entire scene features The Doctor talking to himself (or an unseen Borusa) as he tries to figure a way out of the office. As if talking to himself wasn't enough, after finding that his trusty sonic screwdriver won't open a secret hidden door, Baker full on turns to the camera and says (everybody now), "Even the sonic screwdriver won't get me out of this one". Tom Baker has spoken to three different people in this scene - a cabbage, Borusa, and you, the viewer.

Later on, Baker shares a (somewhat pointlessly long) scene with K-9 inside the TARDIS. It's almost like Baker doesn't want to actually share screen time with fellow actors, content to hog tight two-shots with robotic dogs and invisible vegetables. This story is the most obvious example of the transition of the gloomy, eccentric Fourth Doctor of his first three seasons to the whimsical, dramatically indifferent Doctor of Seasons 16 and 17.