After last week’s slightly slow story, Spooks springs back to form with a relentless pace and one of its finest episodes not just in this series, but its entire ten year run.

Two Muslim extremists, radicalised in prison, are preparing suicide bombings against the capital. Not a groundbreaking plot for the show by any means, but it soon becomes apparent our spies are up against a more cunning foe than the usual terrorist, and one whose ruthlessness means there’s a new twist on the usual suicide bomber plot. Only their inside man, a potential bomber himself, can help them foil an attack.

As MI5’s snitch Ashur Mohali, Asif Khan gives us the stand-out performance of the episode as a man forced to join a suicide mission or pay a terrible price. His face constantly drenched with worry, the personal conflict he goes through is painful to watch and leads to a marvellous unpredictability throughout the episode.

There’s a palpable feeling of dread that hangs over the entire hour, and it never drops. It’s another typical Spooks race-against-time plot but one that actually comes across as being urgent. Right from the opening it starts ratcheting up the tension, up and up, culminating in a genuinely gripping final 15 minutes by a London landmark that will leave you on the edge of your seat and with nails bitten down to the bone.

One particular aspect of the villains’ plan feels a tad rushed on second viewing, and could have been an episode in itself, but the fact that it’s used at all certainly ups the stakes. Brutal, fast-paced and personal, Episode 4 is everything you’ve loved about Spooks for the past decade.

Even the continuing Gavriks/CIA arc has a greater aura of menace to it, in conversations as well as actions. Unsurprisingly the more it unfolds the more personal it becomes for Sir Harry, to the point where he’s starting to put more and more at risk. It goes without saying that once more Peter Firth turns in a faultless performance as Sir Harry, whether trading frosty barbs with Ilya Gavrik (Jonathan Hyde) or longing glances with Ruth (Nicola Walker).

It feels like every one at The Grid has something important to do this week, even the so far ‘useful as a chocolate pistol’ Calum (Geoffrey Streatfeild), and the performances from all are solid as a result. But it’s made clear from the start that this is Erin’s (Lara Pulver) episode, as she takes centre-stage and becomes a little more fleshed out.

Pulver continues to do a great job of portraying an agent who’s ambitious but isn’t quite as tough as she makes out to be; trying to balance the stresses of being a mother with the stresses of protecting the country, and only just pulling it off. Though a strong female presence, she’s still no Roz, and if you haven’t taken to her by now then this episode will do little to change your mind.

Episodes like this show why it’s a shame that Spooks is losing the Sunday night ‘Berettas vs Bonnets’ ratings war against Downton Abbey. Still consistently strong after a decade, at least it looks determined to go out with a bang.

Airs at 9pm on Sunday 9th October 2011 on BBC One.

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