When Varsh and the rest of his gang invade the TARDIS in Episode Two, and then, amazingly, pilot the TARDIS to land directly beside The Doctor on board the Starliner in this episode, it signals one of the great shifts in how one of the great icons in Doctor Who will be treated from here on in - the TARDIS itself.
Look at how sacrosanct The Doctor's time ship was in the early days of the show, and how irritated, almost panicked, The Doctor would become by the very notion of something penetrating the safety of the TARDIS. In fact, it took until Season 3's The Daleks' Master Plan for a non-companion (namely Bret Vyon. Now where's his companion status?) to successfully enter the ship. Even in the years since then, you could almost count on one hand how many others crashed the TARDIS party, even if only for a short while. Only during the early Tom Baker era did The Doctor start inviting people in to his previously private abode.
But now, four kids casually open the door to the TARDIS and tell Romana, at knifepoint, that they're taking over the ship. (As an aside, how did they get in? Did Adric have a key, too? At the climax of Episode Two, Adric slams the outer doors shut, but then tells Tylos to open the doors after chastising him for closing them in the first place!). Looking ahead, this is the first of many occasions of TARDIS abuse that would happen throughout the 1980s as this once mystical time machine became increasingly knocked down a peg or two.