On his second day back at work, Harry is on the Grid by 7 am. The new intelligence analyst arrives, as instructed, at 8 o'clock sharp.

Her name is Anna Moretti. She is everything Harry has dreaded Ruth's replacement being.

She is Ruth's replacement.

She is young – only 28.

She is a graduate from Durham University (Mathematical Sciences, majoring in statistics and probability)

She is ambitious.

She is highly talented and intelligent.

She is Ruth's replacement.

She is arrogant.

She does not yet know her limitations.

She is Ruth's replacement.

She is everything Harry had once been, so he is familiar with her desire to impress, to be seen as the best ever, to reach a stage in her career where she is above being questioned – by anyone.

What is worse, she is the antithesis of Ruth.

She has no humility.

She is as cold as an Arctic wind.

She seems unaffected by criticism.

She thinks kindness is a body of water in Scotland.

He calls a briefing in the meeting room to introduce her to the team, and to find out what she is capable of. When it is Anna's turn to say her piece, she unwittingly does herself no favours.

"As you no doubt know, I'm here to replace Ruth Evershed. I hear she was the best."

What follows is a distinctly uncomfortable silence, as the other members of the team attempt looking anywhere but at Harry. They all know how he's been after Ruth's death. News travels fast in their small world. None of them want to remind Harry of what he's lost, both personally and professionally.

"Yes, she was the best," Harry replies quietly. "You'll never replace her. Don't ever imagine you can."

Anna seems momentarily nonplussed, while Erin, Calum and Dimitri smile to themselves. Serves her bloody right. Best of all, Harry appears to be back on form. The team emit a collective sigh.

After the meeting, Anna seeks out Erin.

"What was that about?" Anna asks. "Why all the weirdness around Ruth Evershed?"

"Ruth Evershed was a genius, but she was also humble, and we all loved her. Every one of us. She died tragically and unnecessarily. Had she not, you'd still be looking for a job. Don't ever think you can replace her. All you can ever do is to aspire to be like her."

The new girl has been told her place, but it is unlikely she'll pay attention.

Her first attempt to cover herself in glory goes a bit tits up.

She uncovers some internet chat about bombings planned for North London. It's all very non-specific, and Harry is not interested. Being used to Ruth's style of checking and re-checking her information, he asks Anna to go back and check her sources.

"The bombers are Turkish, and they're targeting government run schools." Anna insists. "I think we should inform the relevant authorities."

"How sure are you?" Harry asks, aware that this isn't Ruth, and so questioning her is necessary.

"As sure as I've ever been of anything."

"Calum isn't convinced," Harry counters. "He thinks that the information source is unreliable."

"Calum isn't trained in analysis," Anna answers, her irritation beginning to show.

"Calum has been filling in for the past 4 weeks, since Ruth – since our last analyst moved to the Home Office."

"So," Anna smiles as she steps over the line. "What is it about this Ruth? Why was she so special?"

"That," says Harry, raising his voice, like he used to in the old days, the days before Ruth had entered his life and calmed him the hell down, "is none of your damn business!"

The new intelligence analyst leaves his office, and Harry escapes to the roof. Out there, in the cool morning air, he is able to breathe more freely, allowing the anger to leave his body.

"Oh, Ruth," he says aloud, "where are you when I need you?"

I'll never be far from your side, Harry. I'm wherever you need me to be. Think of me and I'll be with you.

Despite a dead stillness in the air, Harry feels a cool breeze touch his face. His skin prickles with her presence. The tension in his body leaves, and once again he feels loved, as opposed to abandoned.

"What am I to do, Ruth? Did I come back too early? What is it with this woman? I don't think I can work with her. And the real reason I can't work with her is because she's not you."

Until I am gone from here – from you – no-one, not even the most competent analyst in the world will satisfy you. Anyone they send you will not measure up to me. That is how you see it. This young woman has to learn the lesson of humility, and you are just the person to teach her that lesson.

"So is that how it works, Ruth? The people who come into our lives are the ones we need most, whether we want them or not. Please don't leave me, Ruth. I don't think I could stand it if you did."

Harry is feeling sad and vulnerable. As much as he is grateful and even happy to have Ruth with him, even in spirit form, he suddenly misses her physical presence. He wants to be able to touch her, even if only fleetingly. He can feel the tears welling in his eyes.

"Are you OK, Harry?" It's Erin. She has managed to step on to the roof without him hearing her.

Harry nods, since he can't trust his voice at this moment. "No, cancel that," Harry adds. "I'm not all that OK. I yelled at the new analyst."

"And you miss Ruth," Erin adds quietly. "We all do, but no-one more than you. You lost the woman you love, Harry. It's going to take you some time for you to come to terms with that."

As much as Harry wants to snap at Erin, he feels he can hardly give her a bollocking for having told the truth. He passes a hand across his eyes, wiping away the tears.

"Dimitri and I are here if you need us, Harry. Let me deal with Anna."

"Thank you, Erin, but I think that I have to deal with her myself. You can't keep me wrapped in cotton wool. I have to face a Ruthless world eventually. I'm thinking that I might just follow the new girl's intel – just this once – and see where it leads us."

"Is that wise?" asks Erin.

"No, it's not wise at all. Ruth would never advise it. But then again, Ruth would never have come to me with such a half-arsed story, either. I'm fine, Erin. I can handle this."

Once Erin leaves the roof, Harry smiles. "This is your work, isn't it, Ruth? You brought Erin up here."

The sudden breeze-from-nowhere again cools his face. He smiles to himself.

Against his better judgement, Harry sends out Dimitri and Justin, another one of the new operatives, to investigate Anna's `information'. Just because he can, he insists Anna accompany them. Before they have driven more than two blocks from Thames House, they are informed of a series of explosions at an address in Hackney. Dimitri plants his foot.

What they find when they reach the address is a bunch of 15-year olds who had paid far too much attention in science class. They'd found a recipe for a bomb on the internet, and surprisingly it had worked without blowing the kids' faces off in the process. But it had scared the neighbours, and it was they who had called it in. The fact that the boys are Turkish is almost incidental to the occurrence, apart from the fears of the non-Turkish neighbours. Harry hands the case on to the police, hoping they will do a good enough job of scaring the pants off the boys, so that he – or anyone else in the security service – will not have to deal with them with more severity some time in the future.

Anna Moretti is humbled, but maybe not enough. There will be considerably more opportunities, but for now, the Turkish boy bombers will have to be enough.

"You knew, didn't you?" Harry said aloud once he was back home in his own house. He felt the tingle on his skin of Ruth's presence. Scarlett stared over Harry's shoulder, but no longer growled when she detected Ruth in the room. "How does this work, Ruth? Can you see into the future? Tell me."

I have no more insight than you do, Harry. It's just that without my body, I seem to be able to tap into another kind of knowledge. Everyone has access to it, but we've been trained to trust our intellects over this other insight.

"So, will you be my guiding light, Ruth? Is this why you're still here?"

Looking back on my physical life, I can now see that my father was guiding me so much of the time. I now believe that he led me to MI-5 and to you. Meeting you, working with you, loving you was one of the most painful, frustrating, but beautiful experiences I had during my life. I have to thank you for that, Harry. Thank you for loving me and protecting me, and you did protect me, so many times. It's now my turn to protect you.

Later, Harry fell asleep knowing that Ruth was with him, protecting him and gently guiding the direction of his life. Whilst he found this comforting, it did not replace her physical presence. Not for the first time, and surely not for the last, Harry feels guilt gnawing away at him. She said he'd protected her, but the one time she'd needed her protection the most, he'd not acted quickly enough. He now is to pay for that oversight for the rest of his life.