Sarah, Harry, and the 4th Doctor investigate a far-future Earth. A good deal of fun (though there's a dark section in which Sarah is tortured in a non-detailed way), but it does contain a particularly terrible technobabble explanation for one of the Doctor's miraculous escapes from certain death:
"'Why weren't you killed when Styr threw you into the ravine?'
'Yes, I thought you might be wondering about that,' the Doctor smiled. 'It was all thanks to this.' He rummaged in one of his inside pockets and carefully took out the small piece of Terullian alloy, gripping it tightly with both hands.
'That?' cried Sarah, frowning in disbelief. 'How on earth could that have saved you?'
The Doctor grinned mischievously at the four sceptical faces around him, obviously relishing their confusion.
'This is a fragment of the Scavenger's levitation system,' he explained, 'which works on much the same principle as the gravity bar. Now, when I poured Styr that wee dram [of whiskey], a drop or two must have got into his control unit and, by a stroke of good fortune, reversed the polarity of the gravitaton fields in this little thing.'
The others stared blankly at the insignificant-looking scrap of metal the Doctor was holding up in front of them.
'So?' Sarah said, after a pause.
'Well, it's obvious,' cried the Doctor. 'This little fragment suddenly acquired an intense dislike for the Earth's gravitational attraction and did its best to escape. Since it was trapped in my pocket, it slowed me down--and broke my fall. Simple really.'
...a stunned silence."