After the departure of Ian and Barbara, the show needed both another action lead and another voice of conscience, and in Steven Taylor it found both. Both a rational man and a staunch moralist, Ian is never slow to criticize the Doctor's behavior or challenge the arguments before him, yet is just as quick to defer to greater knowledge and to wait and observe rather than pursue his case just for the sake of it.

The show's strongest moment is a lost one, toward the end of a season three serial called The Massacre. Over the previous two serials the show had shed three companions, killing two of them in the process. The Doctor and Steven had overseen atrocities on a local and a cosmic scale. Finally over the course of The Massacre Steven had a chance at his own solo adventure -- right in the midst of one of history's politically-motivated culls. Through the story Steven grew close to a girl who might well have become the next companion -- until at the last moment the Doctor refused to help her, and dematerialized with Steven, leaving the horror behind.

For Steven, this action was the last straw -- and he let the Doctor know it. And for a few minutes there, it seemed like he might be gone forever. And in those few minutes, for the first time we see the Doctor broken. All of his past misdeeds come home, he comes to question his own motivations, and he crumbles.

Soon enough, Steven returns -- and on the other side, the Doctor is a new man. Well, sort of. He is humbled, at any rate.

Other than Barbara, no other Companion has had as large a role as Steven in shaping the character who the Doctor would become. While people speak at length of the difficulty that Patrick Troughton faced as the second man to play the Doctor, no one ever speaks of the impossible task before Peter Purves, of filling the roles of both the show's heroes at the same time -- yet he does it, and despite some scripting irregularities he does it very well.