Leonard finds it when he's scrabbling under his bed for that box of comics Sheldon disapproves of. Searching for the sharp corners of that familiar box, his fingers instead close around something sharp and flat. He frowns, pulls out his hand and about a thousand dust bunnies, and finds himself looking at a monarch butterfly. A plastic one, lacquered and on a hairpin.

He doesn't have to search very far into his memory to remember that one night when he and Penny went out to dinner and it was a disaster - he fumbled the plate of chocolate cake into his lap, she knocked the wine over the table and on to the floor, he left his credit card at home, hers was (predictably) declined. They'd ended up calling Howard who took the bus over and had to sit through his company on the way home as Penny's car leaked from the roof over their heads.

But it didn't matter, because as soon as they'd stomped all stained and soggy into Leonard's bedroom and shut the door and Leonard glanced at Penny only to catch the smile on her face...

"Oh, come here," she'd said, and then it was all fumbling on zippers and pulling off jackets and sliding the butterfly-shaped hairpins out of her drenched hair and on to the muddy floor as they tumbled into bed, always like two sides of a magnet drawn back to each other over and over.

Staring down at the butterfly in his hands, Leonard shakes himself and shoves it back under the bed, finds that box of comics and is rifling aimlessly through them when Priya walks through his door and kisses him on the cheek in greeting.