We watch Battlestar Galactica for the space battles and the sudden revelations and reversals, of course, but the question has to be asked: why do we end up caring so much? If it were all about the plot—the genocide, the quest, the politics, the religion and the secret androids—it would be a virtuoso exhibition no doubt, but it would not be the show it is. What Ron Moore has given us is a show with real heart, about deeply flawed characters who are gradually, we hope, learning better, but are perpetually finding ways of seriously frakking up.

So in season four it is not just the search for Earth that matters, but the way it turns Starbuck, for a while, into an obsessed seeker for whom it has become less a possible home than the White Whale of the novel from which she takes her name. It is not just Tigh's discovery that he is the thing he hates most in the world, and that he killed his much-loved wife for treason when he is himself a Trojan Horse, or even his bizarre affair with Caprica Six, that is interesting but that he gradually learns from all of this a path of self-sacrifice that we would never have expected of the drunken, arrogant martinet of season one. It is not just the way that abstract ethical disagreements among the Cylons about the way to treat their soldiers and spaceships become by slow accretions of logic the foundation for treachery, murder, and war, but also the way that the Six known as Natalie convinces her followers to renounce immortality. (And without indulging too promiscuously in my habit of decoding names, is it not interesting that she has a name which relates to birth?)

This is particularly important when we consider the three characters who have, by the mid-season break, become the prime candidates for the last Cylon. To digress briefly on this, we have to assume, for narrative purposes—what we call not cheating—rather than in terms of characterization, that D'Anna is telling the truth when she says that only four of the final five are with the fleet. This leaves us with the alternatives that the final Cylon is either on the Basestar when she says this, or that they are somewhere else entirely. If the latter, they would have to be someone we have never met, or someone who is dead; someone we have never met would be someone in whom we have no emotional investment, and would be bad story-telling. None of the dead characters are terribly plausible candidates—Cain would be, emotionally, a repetition of Tigh; Ellen Tigh is essentially a comedic character who wandered into a tragedy, and for her to turn up as the secret mistress of all would be to turn her husband's journey into a dark joke. Other corpses, Billy for example, are just implausible.

If D'Anna is telling the truth, we can rule out such implausible, but conceivable, candidates as Tom Zarek, Apollo, and the Galactica itself—and Starbuck can be ruled out on the grounds that she is, or is now, after her apparent rebirth, probably something other than straightforwardly human, but not a Cylon. There are four important supposedly human characters present on the Basestar when she says this, and I think we can rule out Helo almost at once, on the grounds that, if he is not human, then his child by Athena is not a miracle baby, but the sort of cross between the two Cylon breeds that the child of Caprica Six and Tigh is going to be. (Assuming that it is not the mutant flipper baby of much fannish comment.) This leaves, as it always left, Bill Adama, Gaius Baltar, and Laura Roslin as the principal candidates.

I have my convictions as to which of them it is, but the important thing is less to pursue this further than to consider how much each of these three has been affected by the events of season four, and the extent to which their emotional journeys are important for the future of human and Cylon alike, no matter which of them turns out to be what.

In a show that is about the relationship between humans and androids, in which one crucial point of view is that children can only come into their own at the death of a parent, the importance of Bill Adama is as a patriarch who has gradually come to terms with his adult children. I use the plural because, as his almost-daughter-in-law, Starbuck can be regarded as the daughter he never had—and in no other sense, since the fact she has slept with both his sons rules out a blood relationship in an American TV show. In a crucial scene in the tenth episode, broken by Tigh's revelation, a sobbing Bill is cradled in the arms of his son Lee, who has grown beyond him to the point of becoming, briefly, the human President.

When he nearly executes Tigh, and risks the existence of humanity on a game of chicken with D'Anna, Lee has become dangerously more than his father. For Bill Adama, being a warrior is being head of a system of loyalties; for his son, it has to do with being a system of laws. One of the strengths of the show is that it is agnostic between these two ways of being. The crucial thing about Bill Adama in season four is that he learns to acknowledge that he needs to be loved, and that he is not ashamed of weakness; Edward James Olmos is magnificent in all of this.

Gaius Baltar has always been one of the most fascinating characters in the new version of the show—never a simple traitor, or tyrant, he has jeopardized humanity through venality, ego, and sexual promiscuity, and has at least once saved it by doing the right thing, and not taking credit for it. Tried for his life at the end of season three, he was acquitted thanks to Lee's argument that he has done no worse than anyone else might have done, an off-hand contemptuous dismissal with which he is condemned to live.

Being Baltar, of course, he makes his shame a system of being; the man who was once the show's only sceptic has become its most fervent believer, indeed, a prophet. He comes up with a set of views that are at once incredibly self-serving and deeply attractive: you saved a wretch like me, he prays to his god over a dying boy, why not save this child who has done nothing? In a show about a war in which both sides have committed or attempted genocide, there are worse and less necessary things in the world than preaching a religion of forgiveness.

One of the crucial moments in the last two episodes is Laura's decision first to allow Baltar to bleed to death, when anaesthesia makes him confess his original betrayal of humanity to her, and then to allow him to live. In one of her visions, Elosha tells her that human life is too sacred to be considered on a case-by-case basis—but there is more to it than that; Laura later sends Baltar to D'Anna to plead for human survival, and she knows and acknowledges that he is the better person for the job. Given how much, and with what justification, she has hated and despised him for months, this is a slightly surprising turn-around; visually, it becomes even more remarkable as the scenes where she dresses his wounds come to resemble Pietas, religious icons of the dead Jesus in the arms of his mother.

Laura knows that she is dying, and it concentrates her mind—her visions tell her that she has little time left to rule and lead, and that she needs to learn to love. At one level, this manifests in the consummation of her long-term chemistry with Bill Adama; at another in her decision to forgive Baltar. And yet she is still dangerously flawed—her stateswomanship can come to resemble a compulsive habit of double-dealing. She never entirely believes in the Cylon civil war and her treatment of the Cylons who seek alliance with humanity is dangerously treacherous. One of the reasons why I am inclined to think of her as marginally the most likely candidate for a revelation in the last episodes of the show is just this—she is the one character for whom the discovery that she is not what she thinks she is would be a genuine learning experience. "What else have you been wrong about?" says Torie to her when the newly revealed Cylon throws her allegiance in her face. It is, after all, not irrelevant that Laura used to be Minister for Education.

In the last three minutes of episode ten, humans and Cylons alike learn a sudden very harsh lesson; they have bickered murderously to the very threshold of Earth, and they find themselves on a planet that is dead and blasted, where no comfort is to be found. Some characters seek each other—D'Anna joins Bill and Laura; Baltar sits in grief; Caprica Six seeks out Tigh; Helo and Athena at least have each other. Earlier, Lee asks "Where do we go from here?" and we are left with no answer. All this has happened before; it remains to see whether it will happen again.

Roz Kaveney is a writer and reviewer living in London. Her most recent book is Superheroes!; other titles include Reading the Vampire Slayer, From Alien to The Matrix, and Teen Dreams.