Natasha's suddenly a kitten — an adorable calico kitten with a foofy tail — and Darcy really, really wants to skritch her ears and see if she'll purr, even if she knows that it would probably end in disdain and sadness.
However, working around the Avengers has taught her that getting out of the extra-dimensional black hole portal-thingy created by an unknown Bad Guy in the middle of her favorite coffee shop always needs to come first. So she tucks kitten-Natasha, who still has the blinky eyes of kitten bewilderment, into the front pocket of the hoodie she'd been wearing when she tripped into said portal and prepares to dive back into the real world as soon as Bad Guy is sufficiently distracted by the hundreds of pounds of muscles and technology currently attacking him.
Except then Bad Guy totally Midsummer Night Dreams them (hah, take that everyone who said an English Lit minor was useless!), and everyone's wandering around gazing longingly into each other's eyes. Except Darcy's learned to duck instead of taking it on the chest/armor/shield, so she misses the whammy; Natasha, safe in Darcy's hoodie, escapes, too. (If you can count having already been turned into a kitten escaping. It's better than being an ass, though, and Darcy's going to scrub that bit of snarky commentary from her mind right now before the Widow hears it, kthnx.)
Which means that Natasha can totally dart out of Darcy's hoodie, sneak up behind Bad Guy, claw her way up his ostentatious (see, English minor! totally useful! her reports are gonna be awesome) cape, bite his ear, and, while he's dancing around trying to shake off a persistent kitten, bat his stone of power thingy from his fake-Egyptian headpiece to the floor, where it shatters into a million billion tiny little pieces.
When it becomes clear that the transformations are a regular thing (there's some sort of super-complicated mathematical equation with a lot of Greek letters and superscripts and accompanying explanations about morphological fields, but, basically, it means the surprise factor is gone) they start working on long term strategies. Natasha says that she's okay with occasionally being a kitten — it gives her a whole new set of abilities and infiltration strategies — but she refuses to be sidelined.
So the question arises of how to transport a tiny creature of fur, claws, and teeth (but lacking an impressive stride) into a battle zone.
Thor proclaims kitten-Natasha a fitting companion for a warrior from Asgard and has a blacksmith add tiny platforms to the shoulders of his armor. When his cape is removed, she can perch there and watch all that occurs.
Of course, sitting on a metal plate attached to someone who regularly plays with lightning is not, as Tony points out, a great idea.
His contribution to transporting Natasha into battle is to make her an Iron Kitten suit that can ride along with his red and gold. She expresses her opinion of this by taking the custom Iron Man figurine from his desk, climbing to the top of the curtains, and then dropping it from a great height. Pepper translates, pointing out that if something goes wrong (he's indignant; they ignore him) Natasha would be trapped, and Natasha sharpens her claws on the couch when he starts drawing up new plans.
Finally, Clint adds a small carrier to his quiver. It's clear, so she can see, and it has a magnetic lock she can undo with a pressure sequence, so she can escape if necessary. Darcy cracks up when she sees it - it looks exactly like one of those vacuum tubes from the bank - but, honestly, thus far it's the best option to get Natasha into the hot zone with some autonomy after arrival.
Darcy's doodling on her iPad when Natasha starts her first post-mission-as-a-kitten report while still being kitten-Widow. And, okay, it's neat that Tony rigged something that responds to the slightest quiver of her whiskers or twitch of her tail, but it's also kind of creepy to hear her voice coming out of Jarvis' speakers when Darcy can see her right there. And then Tony's cackling about having hacked Natasha's mind, and her low growls and Clint's increasingly unfriendly demeanor don't dissuade him. Still, he shuts up when she details exactly how she foiled the latest dastardly plot by infiltrating the secret lair and strategically leaving a fine sheen of cat fur on key electronics.
(Natasha promises that she'll never interrupt Darcy's just this side of safety-hazard hot showers by treating Jarvis that way, but, she implies, it won't hurt Stark to be a little nervous when contemplating exactly how easily she can get through the smallest openings, like the ones into a top secret lab/playhouse.)
Sometimes, Darcy and Natasha sleep together. It's not a Thing — Darcy's last Thing involved entirely too many transatlantic flights after Georgie's firm transferred her to London, until they both decided that two ridiculous schedules plus an ocean to cross did not equal a relationship, and her Thing before that is not to be mentioned, although some of her friends (Clint) still wince in thought of the hangover from the post-breakup tequila crawl — but it's nice and really, really fun. Sometimes it's after a crappy day, sometimes it's after a most excellent day, sometimes it's due to lots of adrenaline, and sometimes it just is.
There's some warm fuzziness there, if warm fuzziness means seeing Natasha when she's not wearing any weaponry (there are at least two knives in easy reach that Darcy knows of, but given that the Widow pretty much is a weapon she's totally not counting those).
But then Darcy's got a headache and bottle of wine after a funeral, and normally this is when Natasha would be making grilled cheese sandwiches, heating Campbell's soup, and reading while Darcy used her as a pillow. But Natasha's stuck in kitten mode, and Darcy's sitting on her couch, staring at her ceiling and the rotating blades of the ceiling fan. Then she feels three pounds of fluff start to walk up her body. She raises her head enough to make sure she's not hallucinating and then lets it thunk back down. Tasha reaches Darcy's neck, licks her chin once, and then curls up on her boobs, a sedate purr rumbling out of her little kitten body.