If you love this show and haven't yet seen it, please don't read my review. No, I'm deadly serious. Things take an irretrievable turn in this story, and it should be appreciated unfettered by my reveals.

After watching Adam Raised A Cain this show is wearing the serious but resolute look of a show that's nearly done, even if there's a chance that it isn't. Instead of the trademark slow burn, the penultimate second season story is delivered with a full head of steam, the brakes off and deadman's curve up ahead.

It begins with Sarah and Derek at the grave of Kyle Reese, which is marked in the same fashion as all those other unknown people who died in 1984 by a simple year marker. As such they don't actually know which is his, but it's enough for them to be there. They're joined by John, who shows them a mobile phone he took from a man killed at the lighthouse, on which is a picture of Savannah Weaver.

This is the first direct connection they've had to Weaver, and John recognises her from Doctor Sherman's office.

Meanwhile, young Savannah is getting into trouble at school for chatting on the computer with John Henry during classtime, the two are developing a strong bond as they relate as ‘children'.

John Henry is spending much of his time considering the nature of his brother, which is where the ‘Cain and Abel' analogy starts to resonate. There's a quite scintillating conversation between Ellison and John Henry where the AI explains how the human brain is an incredible computer but flawed, because when you die there is nowhere to download the data to. The bible solves this with ‘heaven', so in John Henry's mind, heaven has a hardware problem. He's also mindful of this because he clearly fears his brother with try to kill him, and needs to know what his intentions are.

Form here we move to possibly the most chilling sequence this show has ever presented, bar none. It takes place in the Weaver home, a luxurious modern architectural edifice set in its own grounds. Here a child minder attends the young Savannah while she does her sums, when the ‘water man' comes to deliver.

John Henry is chatting via a Bluetooth headset when he sees the man moving effortlessly towards the front door carrying a heavy water bottle. He knows what that man is, and what he intends to do, which begins with shooting the child minder through the head when she opens the door.

At this point the affinity between John Henry and Savannah kicks in, as he guides her around the house evading her would-be assassin. Garret Dillahunt as John Henry is quite dazzling in this scene, just putting the right amount of cadence in his voice that you know he's concerned, but not actually human. At one point Savannah asks where Debbie is. John Henry tells her that she's in the foyer, but that they're not going that way.

Eventually, he leads her to the garage, a possible point of escape, but, alas, the 30ft range limitations of Bluetooth apply and he's helpless.

Just when it looks like Savannah is going to be a statistic, she's grabbed from behind by John Connor!

At this point as was wondering where those I'd put those tranquilisers. If I didn't need them here, I'd be requiring them very soon!

The Terminator enters the garage and attacks John and Savannah who are behind a car. He opens fire and moves in for the kill, before he's then attacked from behind in rapid succession by first Sarah and then Cameron.

John Henry is watching in disbelief as the humans make good their escape while Cameron dishes out some metallic mayhem. But the Terminator disables her momentarily, and collecting his gun, goes in pursuit. He returns to the ground level, where Derek has just entered the building. He shoots Derek through the head before he's even had time to react, and he's dead before he hits the ground.

The lack of any precursor to his death hit me like a freight train; for a minute I convinced myself that he's been injured, but we get a close-up and he's most certainly dead, gone in a blink without any heroics or fanfare. I can't think of any currently running show that has the balls to do this, but this show certainly has.

Cameron throws the Terminator off a balcony, giving time for everyone else to escape and for them to know the ultimate fate of Derek.

With the little girl now missing and two dead people in the Weaver home, a police investigation is soon underway, but John Henry knows what happened and reveals this to Ellison. He does this because he wants to know why Ellison kept the existence of Sarah Connor a secret. Soon John Henry will start keeping secrets, from, of all people, Catherine Weaver.

I'm not going to step through the rest of this story lightly, because I wouldn't add much to the experience. What happens is that Ellison contacts Sarah to negotiate the return of Savannah, which is agreed on the basis of Sarah meeting Weaver. While Savannah is with John Connor she tells him about John Henry, the strange man who is wired to a computer, who looks like Chromartie.

After the exchange takes place, Sarah is arrested by the police, which is most certainly the work of Catherine Weaver, but John and Cameron get free.

In a final scene we see the remains of Derek being buried under a small 2009 plinth, in the same field in which his brother lies.

Wow, heavy and intense stuff. If this show does return for a third season then it's going to be radically different, purely on the basis that we're rapidly running out of characters. I've got a feeling, however, that plenty will be wrapped up next week, in such a way that if the show ended it wouldn't leave much unresolved. The obvious question is where does John Henry fit into the bigger scheme of things? Is he SkyNet or is his brother, or is it a combination of the two?

We know that in the future the machines split into two factions, one ambivalent to and one against humans. Does John Henry represent the code that will split them, and give humanity a chance to win?

I wish that I could answer some of these questions, but I'm sure after Born To Run we'll know, or at least have a better idea. The Sarah Connor Chronicles has been the stand-out show of this last year for me, and I'm feeling slightly depressed that it's nearly over. But that won't stop me tuning in for what's bound to be a spectacular episode.

Read a review of the episode 20 here.