The Android Invasion (Fourth Doctor).
The last appearance of Harry Sullivan and Sgt. Benton and unfortunately they had little to nothing to do. The story is good, though when the first alien first appeared I thought it was a Sontaran, but resolves itself too quickly. I think it would have been interesting if the Androids had been able to establish a larger foothold on the Earth before they were defeated. It's also too bad the Brig didn't show up but he probably wouldn't have had much of a role.
Yeah, my reaction was WTF were they doing? Sure, they may not have realized that it would be there last appearances but it was nice to get them back (particularly Benton) after a break and then have them do pretty much nothing! And no Brig?! It was all very strange. Almost better to have eliminated the UNIT connection completely.