Here’s a tip: if a ghost ever turns up on your doorstep, looking a bit like comedian Russell Kane after an explosion in a charity shop, and tells you that he ‘doesn’t want to cause any trouble’, he’s probably lying. But that’s exactly what happens this week as spectral Seventies toy salesman Alfie Kirby (glimpsed last week) comes calling, telling the gang he’s been sent by Nina.

James Lance gives us the dictionary definition of ‘creepy’ as Kirby, and it’s clear from word one that he has a plan of mysterious purpose to divide the Honolulu Heights gang. It’s a real strength of Lance’s performance that the longer you watch him as Kirby the more unsettling he becomes. What starts out as something akin to a weird 70s children’s TV presenter quickly descends into Hitchcockian loon territory.

He’s barely slithered in before he starts wiggling apart the cracks in the trio’s relationship and winding Annie round his finger with softly-spoken platitudes and hollow sympathy. Whenever he pops up, like a spook laid off from the Rentaghost agency for stealing stationery or spreading rumours about Timothy Claypole, you’ll want to hammer on the TV screen like a trapped spirit and shout ‘Don’t listen to him!’.

Socha, Molony and Crichlow are all, once again, brilliant. You’d think we’d get tired of saying that, but week after week they continue to add definition to their characters with incredible nuance.

As with every other writer so far, Tom Grieves gives each character enough room to explore and approach their humanity from their unique directions; whether it’s Tom’s heart-breaking excitement about having his first ever birthday (God, you just want to give him a cup of tea and a big hug, don’t you?), or Hal getting a moment that’s decidedly Sherlock in its writing and delivery as he probes the Box Tunnel investigation.

If you’ve noticed the nods towards how powerful ghosts can be in the Being Human universe (as vampire Wyndam mentioned in passing to Annie at the end of Series 3) then this is an episode that’ll further stoke your interest in just what spooks like Annie are capable of. Without giving anything away, it’s a LOT more than just providing tea or making lights blink, and we’ve probably barely scratched the surface.

Underneath the main story the Series 4 arc begins to knit together in some rather clever ways that’ll likely elicit an ‘ahh!’ or an ‘ooh!’ or maybe both. Go on, treat yourself to both.

A part of the ‘war child’ prophecy pops up, the mysterious ghostly lady does another ‘Rose Tyler’ through the telly, and Cutler’s plan for vampire supremacy rolls forward. It’s as exciting as it is ominous, and it’s all weaving together in a far neater way than last year’s ‘wolf-shaped bullet’ saga.

Being Human is on fine form, and as we reach this series’ halfway point (already!) we’ve no reason to believe that’ll stop. Based on everything that’s transpiring, we’ve a feeling that the finale is going to be one Hell of a show.

Airs at 9pm on Sunday 26th February 2012 on BBC Three.

> Order the Series 4 DVD on Amazon.

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