Review: Terminator: The Sarah Connor Chronicles 2.21

Like episode 2.20 before it, 2.21 - Adam Raised a Cain is full of brilliance; somewhat lacking in many of Season 2's episodes previously.
Like Battlestar Galactica, this series shows how life can be extinguished in a brief moment with no warning.
2.21 opens with a series of graves marked only by the year the victim died. Both Derek and Sarah are looking at the graves of those lost in 1984. Somewhere amoung them is the grave of Kyle Reese. Derek explains his disapointment at Sarah's treatment of him citing that the grave of his brother should be enough for her to have trusted him. she explains that the only person that really matters is John.
John arrives to show the pair a mobile phone retrieved from a corpse at the lighthouse. It contains a picture of Savannah Weaver who John recognises.
The subject of the photo is getting into trouble at school for communicating with John Henry who the teaching staff assume to be a paedophile.
John Henry is pondering death after his power down and talks with Ellison on the nature of heaven and whay it might have a "hardware" problem. He feels that his "brother" might be trying to kill him. John Henry continues to chat to Savannah via a Bluetooth ear piece as Savannah is drawing in the front room of the apartment. JH sees the "water man" approach as the nanny opens the door and receives an accurate shot to the brain.
John Henry guides Savannah away from the danger as the Terminator scours the building looking for her. Hearing the A.I. showing genuine concern outside of it's normal programming is quite chilling and caused me to think once again as to what JH is. Is he skynet in embryonic form? Or is he a counter Skynet created by Catherine Weaver; a T-1000 sent down the line by John Connor for this very project.
As Savannah appears helpless, after going out of Bluetooth range and seemingly into the cross hairs of the Terminator's weaponry, John turns up to rescue her. In an instant, shockingly, Derek takes a bullet to the head. This happened so fast I assumed he'd been hit in the shoulder but a close up shows the bullet hole in his forehead; totally unexpected with no signpost whatsoever!
Savannah is reported missing. JH consciously neglects to tell Ms Weaver that he has CCTV footage of Sarah Connor and company but instead quizzes Ellison about it, asking him why he lied. he wants to know why Ellison kept the secret of Sarah Connor's existence.
Whilst Savannah is kept safe with John, she tells him of her friendship with John Henry. Cold realisation issues across John's face as he realises what JH could be.
Ellison arranges for Sarah to hand back Savannah in exchange for meeting up. Sarah is double-crossed and arrested. John and Cameron get away.
At the end, Derek is buried in the same area as Kyle.
Great entertainment, with the same amount of nihilism that normally makes up a Battlestar Galactica. It's a pity it's taken this long to get so good.