Much like the Thunder Dome, on Doctor Who when one companion leaves another companion must soon come aboard. In these early days, the swap is clearly in service to future stories already written -- teenager Susan leaves, and so teenager Vicki slots right in. Man of Action Ian leaves, and so Man of Action Steven slots right in. Arguably Barbara fails this mold, to which I would say: 1) Barbara was a goddess, and cannot be replaced, and 2) More likely, whatever future scenes had been written for Barbara could have been picked up easily by Vicki, so the same principle still applies. And so, with today's departure of Vicki also comes today's arrival of Katarina. Of course, Katarina won't be around all that long... But that's a conversation for another day (specifically: Wednesday). But today we have Trojan horses and Achilles tendons to discuss.