Contact has been made.
Early seasons of Doctor Who suffer from the TARDIS problem—with the ability to get the Doctor out of any trouble, through its inviolable walls and inter-dimensional time and space travel capabilities, the TARDIS requires writers to deploy all sorts of legerdemain to take its plot-skewing powers away from him. Only a change in the Doctor’s attitude, from risk-averse observer to inquisitive hero, solves the otherwise intractable dilemma, with the Doctor finally wanting to stay rather than to flee. Veteran writers Bob Baker and Dave Martin introduce another such problem in “The Invisible Enemy” (Story Production Code 4T) through their invention of K-9, an indestructible robot dog with formidable offensive capabilities and a databank rivaling that of the TARDIS itself. As will become all too common over the next four seasons, the solution here is to have K-9’s batteries run out right when its firepower is needed most. Luckily K-9 has a tow-hook welded on front.
Which is not to say that this four-part story, focusing on an invasion not from without but from within, lacks for action. Leela racks up her highest body-count to date, K-9 zaps quite a few more, and the Doctor himself even blows up a giant base, all to stop the predations of a long-dormant, sentient virus disturbed from its slumber in deep space by a passing Earth shuttlecraft en route to Titan. Before it lands, all three members of the crew have been “contacted” by the Swarm, and they set about converting the refueling installation on Saturn’s largest moon into a hive that will serve as breeding grounds for future generations of the virus.
The TARDIS also stumbles through the viral cloud, and somehow the core intelligence of the virus, the Nucleus, penetrates the shields, interfaces with the TARDIS itself, then passes into the Doctor, who promptly collapses. The virus then attempts to convert with Leela, but for some reason she is immune. With the TARDIS coordinates already set for Titan after the Doctor and Leela received a mayday from the base’s unconverted crew, Leela manages to materialize the ship and awaken the Doctor, who assures her that nothing is wrong. But contact has been made…