John glanced across at Mary as she slept on the plane. He thought back to the woman on the bus – E. He thought about the daily texts and subtle flirting, the mixture of guilt and excitement he had grown to somewhat enjoy. As his wife's eyes twitched in sleep, he felt like he had married a stranger. Yet somehow, he loved her. He loved her so much that he had stopped talking to E, no matter how much he wanted to. He had put time into rebuilding their marriage. He had flown to Morocco without sleep just to bring her back to him.

They stepped into their house, throwing down their bags and turning to each other with solemn eyes.

"Please don't ever do that again," John said softly. "I know you wanted to protect us, but 'the problems of your future are my privilege', remember?"

Mary nodded, biting her lip as her eyes welled up.

He pulled her into a hug. "It'll be okay," he whispered. "It'll be fine."

III

The mid-afternoon sky was overcast, with dull grey clouds appearing even duller through the London smog. Sherlock pushed his hands in the pockets of his coat as he walked along Vauxhall Bridge. He stopped in the centre and glanced over at the murky water. He was thinking. As Sherlock often did.

Margaux rushed down the street to meet him. She hurried up to him and wrapped her arms around his waist, burying her head into his chest. Sherlock stood with his arms by his side, fighting a smile that was trying to force its way out.

"I've only been gone for 48 hours," he said.

She looked up at him. "Are you purposely leaving out the part where you fought off an angry, gun-wielding super agent?"

"Ah so you've already spoken to John." He looked down at her, assessing her face for a moment. "Oh wait, no, it was Mary. You visited her this morning. She made you a cup of tea but you didn't drink it."

"How do you do that?"

He let out a small laugh and backed out of her embrace, turning to look out across the water again.

"How is she?" he asked.

"She tries to pretend she's okay. But I know she's shaken by it all." She joined him at his side, folding her arms and resting them on the bridge. "At least that A.J's out of the picture. She can relax now, get back to normal."

He made a deep, gravelly noise in his throat.

She looked up at him. "Or not?"

"In Morocco, A.J said something… He said the English Woman betrayed them. He thought it was Mary but it wasn't." He put his hands back in his pockets. "There was a code word that this woman said to make the hostage-takers aware that AGRA were coming. Ammo."

"Ammo?"

"I thought about it for a while. Then something clicked. Lady Smallwood."

"You think Lady Smallwood was the English Woman?"

"She was the conduit for AGRA – gave them their assignments. And six years ago she ran an operation with the codename 'Love'… Amo is Latin for love."

"Bloody hell," she sighed. "I assume you've told Mycroft."

His eyes narrowed. "I watched him interview her."

Margaux saw the conflict in his expression, the uncertainty tucked away in the lines around his eyes.

"It wasn't her." She nodded.

"No."

"Mm, too obvious isn't it." She glanced up at him. "Things like that; betrayals, double-crossings, they always come from people who you'd least expect."

"Are you profiling the English Woman?"

"Would you like me to?"

"I suppose it wouldn't hurt."

"Okay… Amo – it's quick to say, has many different meanings so it's inconspicuous, even in a room full of people. Makes me think the woman spent a lot of time in the company of others. A knowledge of AGRA and the hostage situation means she had access to very classified information, so I think you're right in your assumption that she works for the government. Just not in a position of power. I wouldn't be surprised if she does something mundane as a way of appearing powerless to avoid detection – a driver, a secretary, a cleaner–"

"What?" he turned to face her, his movement so sharp it almost startled her.

"What? I forgot what I said."

A memory flooded Sherlock's mind; Oh, you'd be amazed at what a Receptionist picks up. Mary had said. They know everything!

A receptionist.

"Vivian Norbury," he said.

"Who?"

He grabbed her face in both hands. "You're brilliant." He let go and began to walk away.

"Sherlock, we talked about walking away without saying goodbye…"

"Right, yes." He approached her again. "Goodbye."

"Goodbye." She smiled.

III

As Sherlock walked through the London Aquarium, he remembered a school trip when he was six. He remembered making the tour guide angry by correcting his facts, making his teachers worry by wandering off when he got bored. He remembered standing in a tunnel made of glass, feeling like he was inside a snow globe as he watched the fish swim slowly over his head in the glittering water – like someone could pick him up, shake him and he would float there.

He stepped into the tunnel, noticing how much smaller it felt. The water above him didn't sparkle like it seemed to when he was six. He wondered if it ever did, or if his view of the world had dulled as he got older. He turned the corner into a secluded room with floor-to-ceiling tanks. The glow from the water shone a deep blue light across the room, shadowing a woman sitting on a bench. Vivian Norbury.

"Your office said I'd find you here," he said.

"This was always my favourite spot for agents to meet. We're like them: ghostly, living in the shadows." She turned to look at him.

"Predatory."

"Well, it depends which side you're on. Also, we have to keep moving or we die."

Sherlock looked around the room; at the sharks gliding through the water, the fluorescent jellyfish bobbing past the glass. "Nice location for the final act," he said. "Couldn't have chosen it better myself. But then I never could resist a touch of the dramatic."

"I just come here to look at the fish." She stood up and walked closer to the tank. "I knew this would happen one day. It's like that old story–"

"I really am a very busy man. Would you mind cutting to the chase?"

"You're very sure of yourself, aren't you?"

"With good reason."

"There was once a merchant in a famous market in Baghdad."

Sherlock closed his eyes. "I really have never liked this story."

"I'm just like the merchant in the story. I thought I could outrun the inevitable. I've always been looking over my shoulder; always expecting to see the grim figure of–"

"Death?" Mary Watson marched into the room, stopping at Sherlock's side.

"Hello, Mary," said Sherlock without turning around.

"Hey."

"John?"

"On his way."

"Let me introduce Amo."

Mary glared across at the frail old woman, her eyes narrowing. "You were Amo? You were the person on the phone that time?"

"Using AGRA as her private assassination unit," Sherlock added.

"Why did you betray us?"

Vivian shrugged. "Why does anyone do anything?"

"Oh, let me guess," he replied. "Selling secrets?"

"Well, it would be churlish to refuse. Worked very well for a few years. I bought a nice cottage in Cornwall on the back of it. But the ambassador in Tbilisi found out. I thought I'd had it. Then she was taken hostage in that coup," she laughed. "I couldn't believe my luck! That bought me a little time."

"But then you found out your boss had sent AGRA in."

"Very handy. They were always such reliable killers."

"What you didn't know, Mary, was that this one also tipped off the hostage-takers."

Mary's eyes widened as she looked up at Sherlock.

Vivian sat down, resting her handbag on her lap."Lady Smallwood gave the order, but I sent another one to the terrorists with a nice little clue about her code name should anyone have an enquiring mind. Seemed to do the trick."

"And you thought your troubles were over," said Mary.

"I was tired; tired of the mess of it all," she sighed. "I just wanted some peace, some clarity. The hostages were killed, AGRA too… or so I thought. My secret was safe. But apparently not. Just a little peace. That's all you wanted too, wasn't it? A family, home. Really, I understand. So just let me get out of here, right? Let me just walk away. I'll vanish. I'll go forever. What d'you say?"

"After what you did!?" Mary bounded towards the old woman in rage.

Sherlock followed. "Mary, no!"

Vivian stood up quickly, pulling a gun from her handbag and pointing it at Mary.

Mary stopped suddenly and began to back away. "Okay…" She returned to Sherlock's side, staring down the barrel of the gun.

III

Margaux stood at the photocopier, tapping her nails against it impatiently as she waited for it to spit out her paper. She glanced across the bullpen to Lestrade's office, watching as he hurried out of the room and locked the door behind him.

"I know that look," she said as she caught up to him.

He threw on his coat as he rushed quickly down the corridor. "Ey?"

"That's the 'Sherlock bloody Holmes' look."

"Actually this time it's Mycroft bloody Holmes as well."

"Mycroft?"

"Just got a call from him telling me to meet him at London Aquarium." He stopped at the exit. "And to bring a team of officers. John called him." He pushed down the bar to open the door.

"John called him? Oh no," Margaux sighed. "AGRA."

"What?"

"Mary. This is about Mary." She turned around and hurried back down the corridor. "Go on, I'm just grabbing my car keys and I'll be right behind you."

"You know Mycroft specifically said 'don't bring Margaux'…" Greg called after her.

"Ah that's okay, Mycroft knows I don't listen to him."

"Fair enough."

She held the phone to her ear with her shoulder as she ran across the carpark, unlocking the car clumsily and climbing inside.

"Hello?" the voice on the phone played through the speaker as the car came to life.

"Molly, can you hear me?"

"I can hear you."

"Okay, listen, something's come up. I was wondering if you could possibly pick up Vaughan from nursery for me?"

"Oh, erm…" Even through the phone, Molly's apprehension was clear.

"What's wrong? Are you busy?" Margaux began to drive as quickly as London traffic would allow "It's okay if you're–"

"No, it's just that John asked me to babysit Rosie. I'm heading over there now."

"Is he going to the aquarium by any chance?"

"I don't know. He just said something came up... the Aquarium?"

Margaux sighed, butting her head against the steering wheel in frustration. "Molly, look I know I'm a pain but is there any way you can pick up Vaughan and take him with you? I'm so sorry, I'll explain later."

"Okay... Margaux, is everything alright?"

She could hear the sprig of worry in Molly's voice. She took a deep breath and smiled as she spoke.

"Everything's fine. Well actually no, I don't know if everything's fine. But I'm going to find out."

III

Vivian looked down at the gun in her hand. "I was never a field agent. I always thought I'd be rather good."

"Well, you handled the operation in Tbilisi very well," said Sherlock.

"Thanks."

"… For a secretary."

"What?"

"Can't have been easy all those years, sitting in the back keeping your mouth shut when you knew you were cleverer than most of the people in the room."

"I didn't do this out of jealousy!"

"No? Same old drudge, day in, day out, never getting out there where all the excitement was. Just back to your little flat on Wigmore Street."

Her mouth opened in shock.

Sherlock gestured to her dusty shoes. "They've taken up the pavement outside the Post Office there. The local clay on your shoes is very distinctive. Yes, your little flat."

"How do you know?"

"Well, on your salary it would have to be modest and you spent all the money on that cottage, didn't you, and what are you, widowed or divorced?" He looked down at the gold band on her index finger. "Wedding ring's at least thirty years old and you've moved it to another finger. That means you're sentimentally attached to it but you're not still married. I favour widowed, given the number of cats you share your life with–"

"Sherlock…" said Mary as she watched Vivian closely.

"Two Burmese and a tortoiseshell, judging by the cat hairs on your cardigan," he continued. "A divorcee's more likely to look for a new partner; a widow to fill the void left by her dead husband."

Mary's heart began to thud. "Sherlock, don't."

"Pets do that, or so I'm told, and there's clearly no-one new in your life, otherwise you wouldn't be spending your Friday nights in an aquarium. That probably accounts for the drink problem, too: the slight tremor in your hand, the red wine stain ghosting your top lip. So yes. I say jealousy was your motive after all – to prove how good you are, to make up for the inadequacies of your little life."

Mycroft walked into the room followed by Greg and his three officers.

"Well, Mrs Norbury. I must admit this is unexpected," said Mycroft.

Margaux walked in quietly, trying to hide her breathlessness from running. Sherlock turned his head, narrowing his eyes and furrowing his brow; silently questioning what the hell she was doing there. She widened her eyes and shook her head, willing him to turn his attention back to the woman holding the gun.

"Vivian Norbury," Sherlock said, turning back to her. "Who outsmarted them all. All except Sherlock Holmes." He took a step forward. "There's no way out."

"So it would seem." Vivian gave a gentle smile. "You've seen right through me, Mr Holmes."

"It's what I do."

She tilted her head to the side. "Maybe I can still surprise you." She aimed the gun at Sherlock with a shaking hand.

Margaux gasped quietly, instinctively taking a step forward.

Lestrade put his arm out to stop her before turning to Vivian. "Come on. Be sensible," he said.

"No," replied Vivian plainly. "I don't think so…"

Sherlock watched as a flash of light burst out the end of the gun, yet before the loud bang could ripple across the room, Mary was in front of him. He felt her elbow in his chest as she pushed him back, his arms flailing to catch his balance as he watched her drop to the floor.

"Surprise," said Vivian spitefully as two police officers grabbed her by the arms and dragged her away.

Sherlock dropped to his knees beside Mary, pressing his gloved hands against the bloody wound on her chest. "Everything's fine. It's going to be okay." He turned to Mycroft. "Get an ambulance."

Margaux stood frozen in panic, taking quick, shallow breaths as she watched them on the floor.

"It's all right, it's all right," said Sherlock softly.

John rushed into the room. "Mary!"

"John!" cried Mary through heavy breaths.

Sherlock stood up and took a step back as he watched on.

John pushed his hand against the wound, holding the back of her head with his other hand. "Mary? Mary? Stay with me. Stay with me."

"Oh, come on," she replied.

"No, don't worry. Don't worry."

"Oh, come on, Doctor, you can do better than that." Her voice broke with a mixture of pain and fear.

"Come on, Mary… Mary, come on."

"God, John, I think this is it."

"No, no no no, it's not."

"You made me so happy," she smiled. "You gave me everything I could ever, ever…"

"Shh, shh."

"Want." She finished, her body beginning to shiver.

"Mary, Mary…"

Sherlock glanced across the room to Margaux in shock. She looked back at him, silently pleading with him to do something. Anything. But he shook his head gently. There was nothing he could do except stand there. He had never felt so helpless.

"Look after Rosie," said Mary. "Promise me."

"I promise."

She began to cry. "No."

"Yes, I promise," John replied, louder this time.

"Promise me."

"I promise. I promise."

She stroked his face before looking up at Sherlock. "Hey, Sherlock."

"Yes?"

"I… so like you. Did I ever say?"

Mycroft hurried back into the room with his phone in his hand.

"Yes." Sherlock smiled, his eyes glossy. "Yes, y-you did."

"I'm sorry… for shooting you that time. I'm really sorry."

Sherlock forced a smile. "It is– it's all right."

"I think we're even now, okay?"

"Okay."

She looked over to Margaux. "Hey Marg… You make sure you wait. Like we t... Like we talked about, okay?"

Margaux laughed softly as tears escaped her eyes. "Okay."

Mary cried out in pain before turning back to John.

"You…" she sobbed. "Were my whole world. Being Mary Watson… was the only life worth living."

"Mary." John's voice was nothing more than a whisper.

"Thank you." Her head dropped suddenly, her body slumping in his arms.

John drew in a breath. "Mary?" He placed his fingers against the pulse point on her neck before checking her eyes.

She was gone.

The room plummeted into silence as John cradled her lifeless body. Margaux crouched to the floor, covering her mouth with her hands as tears streamed quietly down her face. Mycroft and Lestrade turned to each other. Lestrade shook his head and rubbed his fingers over his chin. Sherlock stared down at his friend. He couldn't move; paralysed with shock and disbelief. This had to be part of a bigger game. A trick. Surely it was just a trick. He waited with shallow breaths for Mary to sit up and laugh, for John to point and say 'got you' before commenting on the look on his face. But they never did. Instead, a low, guttural growl filled the room. Growls of pain and despair muffled against Mary's unmoving chest. Sherlock shuffled forward when suddenly, John's eyes shot up towards him.

"Don't you dare," he snarled, taking in sharp, angry breaths. "You made a vow. You swore it."

Margaux, Mycroft and Lestrade glanced in Sherlock's direction, watching as he backed away slowly. His eyes wide, his chest rising and falling heavily.

John began to sob as he stroked her hair gently. "Mary…"