Most Londoners hate the long evening crush, but Mel found it gave her time to think. On the Tube, she put thoughts of the computer labs behind her, and turned her mind to her thesis. Time was running out. Debt was piling up.

A telltale cloud of perfume wafted by, top-notes strident as an early warning system, doppler-shifting closer.

Mel settled back to work out the best algorithm for -

"Mel, you little sweetie, I nearly blew straight past you there. Can you believe it? He set his flowers on me!"

Perfume billowed. Mel's concentration snapped.

Shauna. That pest of a woman was sinking on to the seat right next to her, breath as harsh as a keel beaching on gravel. Her face was recognizably histamine-puffy, but the swelling couldn't mute her tone of righteous offense.

Was nowhere safe?

No sooner had Shauna taken the apartment upstairs than she’d started calling by daily, installing herself in Mel's kitchen, bitching about English cookies even as she ate Mel right out of custard creams. Even now, the chapter on protein folding was cut off in mid-sentence where Shauna had -

"Mel, I tell you that cracker, Mitch." Outrage, allergy and mascara mingled in her ready tears. "That flake set his flowers on me! Did I deserve this, honey. Did I deserve this?"

Mel blinked. "He did what?"

Mitch was Shauna's son-in-law. He seemed to be the only other person Shauna knew in London. Shauna had assured Mel that she had moved to London from Florida to "be there" for the poor bloke. Mitch's shocking deterioration - he happily ate Safeway Chicken Tikka - was a foregone conclusion after Shauna's undaughterly youngest had made herself scarce on a three-year-contract to Japan, just as Mitch had taken up a post over here.

"Just look at the state of me!" Shauna's blotches were fading, but she looked like hell.

Mel offered her last tissues.

The gaps between Shauna's bloated eyelids narrowed further. "You know where I went wrong?" She took the whole wad. "Too kind-hearted. That's where I went wrong. How many women would have taken the trouble I did for that boy?" She delved into her bag, and pulled out a photograph. A young man and woman stood on a snowy hillside, smiles vivid but eyes straining in the brightness.

"That's Mitch and Amber. I took that on their honeymoon. Not that I stuck my nose in, you understand. Just dropped by to see if I could do anything. Just as well. They were paying a fortune for a lousy room right over the hotel laundry. Hot as a sauna. And noise! Something awful. No, I made a complaint for them, and stuck around until they found them a decent room. Only took a couple of days." Shauna gave a deep wheeze of satisfaction.

"You really should buy recycled tissues," she added.

"I usually do," Mel protested and stopped, furious with herself. Like Shauna cared. She just liked to get to you. As Mel forced herself calm, more commuters squeezed in until the air seemed thick as syrup. "But, hey, Mitch set his flowers on you?"

Shauna's chest heaved.

"This is just the total finish," she said. "The things he's been telling me. I just don't buy it. You know he has a tiny apartment, nothing but a balcony?" Mel grunted encouragement. "Well, first it was just that bonsai Amber gave him. But now, he's got all these mini food plants. Baby caulis. Eggplants like grapes, and some of them are this weird lilac color. And sweet corn, this big," she gestured and eyed Mel's tapered fingers. "Has anyone ever told you, sweetie, you could do with a few extra pounds?" Mel clenched her jaw. "Not a bite on any of them. Total waste of time."

"They sound gorgeous," said Mel.

Shauna hmmphed. "If you like that sort of thing. So anyway, he's telling me, last month he says, 'Shauna if you're gonna smoke, can you do it away from the plants?'"

"He - You're not saying passive smoking can damage plants?" As Mel had learned to fill in Shauna's gaps, she had deduced that Mitch was actually levelheaded and very bright. He worked on plant genetics and clone propagation. Curious.

Shauna produced a knowing smile. "He's crazy," she said with slow satisfaction. "He's totally lost it. 'Hey, Shauna,' he says. 'I've got some slow germinators dormant in that pot. They'll get the best start if the seeds don't get any smoke until the days are getting longer.'"

Mel digested this. Not outlandish at all. Many seeds needed to grow in the aftermath of fire. So smoke was the trigger. "And did you?"

"Did I what?"

"Keep your smoke away from the seeds?"

Shauna smiled so broadly, her eyes disappeared entirely. "No way," she said. "I blew that smoke right over that tub of dirt."

Mel winced, imagining frail, precious seedlings, blanched as bean shoots, stretching up and collapsing in their search for winter sun. A businessman trod on her foot and it throbbed.

"Hey, it was just a tub of dirt, Mel sweetie. Lighten up. You know your problem?" Mel had heard enough versions of her problem from Shauna. She tuned out. Shauna was her problem.

If Shauna stuck around, Mel could just kiss her Ph.D. goodbye. She had tried allowing Shauna just half an hour, but, short of hiring a crane, once Shauna had settled in, there was no way to shift her.

Even the dread that Shauna was about to descend on her had started to sap Mel's energy. A sense of that fulminating presence would sizzle down through the ceiling and eat away her attention. She needed a Shauna-deterrent, even more than a certain marriage she could mention.

Shauna was back on Mitch's case.

"Take last week," she said. "He said to me - and Mel, he's a bit shy of getting to the point, as well he might be. 'Shauna,' he says, 'can you leave off wearing perfume around my plants?' And I just look at him, so he says, 'Well, can you leave off wearing that particular perfume?'"

Mel perked up. "Don't tell me, his aubergine has a thing for you."

"Honey, he gave me this whole load of BS about methyl jasmonate. Says floral perfume - and mine in particular - is based on mee-thyl," she mocked each syllable, "jazz-mohn-ate, and some other stuff I forget. Seems like," Shauna leaned closer in conspiratorial scorn, "to a plant they smell just like another plant someplace close got wounded and there's kinda, blood in the air."

So Shauna really was a bleeding nuisance! Mel cut off a chortle. "Go on. So what do plants do when their neighbors are under attack?"

Shauna frowned at her suspiciously. "Give off 'volatile substances' to repel pests, according to the boy. He said that the more effort his plants put into defense, the poorer they grew. He said" - and her face grew indignantly pink - "I was bad for them."

"Ah," said Mel as light dawned. She examined Shauna's face. The bloating had gone now, and Shauna's wheezing had died down. "And you said."

"I said, 'No way!'," Shauna asserted, and crossed her arms. "It's a woman's right. Honey, this perfume is me.' And he says, 'That's too bad,' but he could get by. Says, 'Thanks for all the baking.' And he showed me the door!"

"This was last week?"

Shauna nodded. "Ten days or so. Course, I wasn't gonna let that stop me. So today, I dropped by while he was out." Mel made a mental note never to give Shauna a key. "And he had new plants there. I don't recall how they looked except some of them had bitty white flowers. Or pastel pink. Or could be the pale patches were on the leaves. Anyhow, plants on the fridge, plants everyplace you could see. Not so big but masses of the little horrors. And I start sneezing the minute I lay eyes on them."

Mel's hands were at her mouth. Of course! Mitch would know about Shauna's allergy to cat dander in every detail. Increasingly often this past month Mel had considered getting a cat herself, but it wouldn't be fair on the poor creature.

"I got the food into the fridge," said Shauna. "But by the time I sorted his mail, I could hardly see. And my chest." Shauna pressed it. "Well, you surely heard it. I coulda been dead on his carpet."

"That's appalling, Shauna," said Mel sincerely. "But he wasn't expecting you. Maybe he forgot you had a key?"

Or never knew.

Shauna's gaze slid away. "I tell you he's done it now." After her recent brush with death, she was now fully restored by indignation. "That's the last that two-bit cracker is gonna see of my Jell-O salad."

"You tell him," encouraged Mel. Her gloom evaporated. She couldn't stop grinning. "You just tell him." Such an ingenious, handy remedy.

A plant, precisely engineered to trigger Shauna's allergy by reacting to her perfume.

"Um, and, Shauna?" Mel imagined an innocuous-looking houseplant cascading over her doorbell. Maybe another standing sentinel in the hall.

"I think -" The train slowed and they both rose to leave. "Oh, I think I might just have a word with Mitch, too."

The doors sighed open and in blew a breeze of cool, refreshing air.