Alex's favorite episode is, "A Good Man Goes to War." In this episode, the Doctor's companion Amy Pond has been abducted by some of the Doctor's enemies. They have stolen Amy's child and intend on raising her to be an assassin against the Doctor.
The Doctor, generally speaking, is extremely non-violent. He does not use guns or weapons, but rather relies on his wits and persuasion to make things turn out well. This stems from a past in which he was involved in a brutal war and made a terrible choice which killed most of his race, in order to also destroy the universe's most evil alien race.
He is so averse to violence now that there are few times do we see him as angry as when his friends or allies resort to violence to solve a problem. As the Doctor said in one episode, "Don't you see? Violence doesn't end violence. It extends it."
This comes into play during A Good Man Goes to War. Seeing no option other than a massive assault, the Doctor starts assembling an army out of people whom he has helped and attacks the asteroid where Amy is being held.
In a key scene, he comes face to face with Amy's kidnapper, and the two have the following exchange:
The Doctor: "I'm angry. That's new. I'm really not sure what's going to happen now."
Madame Kovarian: "The anger of a good man isn't a problem. Good men have too many rules."
The Doctor: "Good men don't need rules. Today is not the day to find out why I have so many."
You know what? The Doctor is right: good men don't need rules. We weren't given the Law by God because we were such good men: we were given so many rules precisely because we are not good men. We are capable of extraordinary evil, and therefore we are given a great, great many rules.
Indeed, every ancient religion saw this. Every ancient religion, whether polytheist or monotheist or philosophy, regardless of the continent, does the same basic two things: (1) define rules to live by, and (2) provide some form of sacrificial system to atone for after you break one of those rules.
This is what is so wonderfully subversive about the Gospel of Christ. He calls us "good men" and removes these rules--long before we have done anything to earn such a title. (In theological terms we call this "imputed righteousness"...we are treated as good even though we have not yet shown it, having received an alien-to-us righteousness of Christ.)
Being (in God's eyes at least) "good men," our rules (the Law) are removed from us. We no longer, like the Doctor, have to have strict rules in place to protect us from ourselves.
Now, many people freed in this way respond to grace in one of two equally wrong ways: they either turn to hedonism (using freedom from the Law as an excuse to live in continual sin), or they return to works (creating a new set of Christian Law to define "goodness"). For the wrongness of the former, see Eph 5:1-20; for the wrongness of the latter, see Gal 3:1-22. Both these were written by the same man, and he exposes the flaws in both lifestyles.
How then shall we live? We live not by trying to create a list of works but rather by cultivating the qualities of a godly life. We seek not to be able to list all the great things we've done (Matt 7:21-23), but rather to become people who live a quality of life like Jesus--specifically, a life defined by love, joy, peace, patience, kindness, goodness, faithfulness, and self-control (Gal 5:22-23). These are not specific works done or undone, but rather values of living that define how we react in situations.
Remember, good men don't need rules--and the God of the Universe decided that all believers are "good men." Further, He promised that He gave you the qualities of a good man (2 Pet 1:3). Who are you to argue with Him--either by starving the qualities He gave you in hedonism, or blaspheming Him by finding them insufficient and thus in need of having more rules added?
You don't need rules. You are a tree planted by living water (Ps 1:3), who has all the qualities of Godliness (2 Pet 1:3), and all you need to do is get your sinful nature out of the way and let Him carry out the good work He has already begun in you (Phil 1:6).