Imagine a drama where dreams never come true; where the characters are real and venal and grasping and even those who wish to do good most frequently end up doing bad; where even the best intentions lead to disaster and long-held hopes of redemption fade a little more every week. Welcome to the world of Battlestar Galactica, America's most depressing television drama.

That's not to say that BSG isn't great. It is. It is dark and twisted and melancholy and frequently quite brilliant but it is also nearly impossible to get through without several strong drinks to numb the pain.

Take the opening episode of the second half of the show's final season, which began three weeks ago. At the end of the first half of season four of our intrepid band of pilots and politicians, Cylons and crew had finally achieved their long-cherished dream and made it back to the mythical planet Earth. And, in a twist typical of this show's dark vision, Earth turned out to be a complete dump. Not a Wall-E style abandoned shell where regeneration might still be possible but a burnt-out husk of a planet where nothing would or could ever grow again. It wasn't simply abandoned – it was dead.

This depressing scenario established, we instantly picked up the final half of the season where we left off, waiting to see how our heroes and villains dealt with their discovery. And the answer was … not very well. We were treated to breakdowns and bewilderment, to rage and impotency, to an act of pyromania on behalf of Starbuck and to Dee's devastating suicide.

It was this last act which serves to illustrate just how different BSG is from other shows. Throughout the episode we watched Dee struggle to cope with the death of everything she had dreamed of. We saw her contemplate life with Lee and ultimately decide that that life would never be the loving, equal pairing she hoped for, we saw her move inexorably towards the decision to take her life. And it was unbelievably painful to watch. Painful because Dee was one of BSG's few genuinely good characters certainly, but painful also because her actions, while shocking, were also entirely in character. Watching her final moments you felt that she could truly conceive of no other way out and, perhaps more importantly, that her actions were terribly, movingly human.

Yes, there are other dramas that turn their spotlight on humanity in all its flawed glory: the bleak and bittersweet The Wire, the ambiguous The Shield, the darkness that lurks at the core of The Sopranos or Deadwood. Yet great though all these shows are, none of them have BSG's relentless refusal to temper or lighten the load. The Wire gave Bubbles redemption and McNulty a rueful escape. The Sopranos and Deadwood allowed wisecracks amid the carnage and even The Shield, whose ending has yet to air in the UK, has been known to show characters a way out of the mayhem and lies.

Not BSG. Three episodes in and, in addition to suicides, breakdowns and a pervasive feeling of general despair, we now have mutiny, destruction and Saul and General Adama facing their own private Alamo. "Trust no one" is Battlestar Galactica's motto. Well, either that or "No one here gets out alive".

And that is the most interesting thing about this show. Not since the sadly cancelled prison drama Buried or Peter Kominsky and Leigh Jackson's brutal, brilliant war drama Warriors has a show been so determined to look at humanity's flaws without flinching. Yes, the end result is depressing and harrowing and seemingly determined to show us humanity at its venal, self-serving worst but the key to BSG's brilliance is that it remains true to that vision, no matter how grim. And that's ultimately why America's most depressing show is also among its very best.