Here is a chapter that, if we had spent the entire fic in Penny's head, would have been called "Well That Escalated Quickly" instead of chapter eighteen.
Penny entered the lobby, Ada settled comfortably into the sling, and smiled when she saw Amy at the mailboxes. "Hey, Ames," she said cheerfully, withdrawing her key from her purse.
"Oh, hello," Amy said unenthusiastically, stuffing an envelope into her pocket and turning away toward the stairs.
Penny frowned. "Is everything okay?"
"Perfectly fine," Amy said, turning to face her again. "You?" she asked, sounding not at all like someone who was asking out of genuine curiosity.
"Good," Penny said, "really good, actually. I'm all done with counseling, and now that Leonard and I are planning the wedding…" she smiled, inserting the key into 4B's mailbox. "Everything is finally going right."
"Hmmm. That must be nice," Amy said, pivoting on her heel and heading for the stairs again.
Penny frowned. "Is something wrong?"
"Yeah," Amy said before the wall hid her from view. "My uterus."
Penny frowned, half at Amy's comment and half at 5B's key being stuck in 4B's mailbox. She yanked on the piece of brass, and it popped out. She shoved it in her pocket and, supporting Ada's head, jogged for the stairs, catching up to the marching Amy on the second landing. "Amy…are you okay?"
"Why wouldn't I be?" Amy said, keeping her eyes forward as they ascended toward the third floor. "It's not like I have any problems in my life." She made a sharp turn to head to the next section of stairs and jogged up the first few, trying to put distance between herself and Penny.
"Um…" Penny wasn't sure how to respond to her friend's sudden chill, and she rushed to catch up. "Is something the matter? Did you find out something else? Are you okay?"
"Oh, I'm fine," Amy said, climbing the final few steps to the fourth floor. "In fact, I'm a lot like you. I have a husband who loves me and a place to stay. But lucky for me I have a condition that renders me incapable of going through the biological process of childbearing. Because I hear that sometimes, the price of these little people loving you unconditionally, these little people that are half yourself and half the man you love more than life itself, sometimes you don't have perfect deliveries and sometimes you get sad that you aren't this powerful baby making machine that never has complications and you have to go talk to some therapist because you somehow don't think your life is just as amazing and perfect as you want it to be."
Penny stopped, staring at the neurobiologist. "Is that what you think of me?" she asked incredulously. "You think that I'm selfish?"
"You are selfish!" Amy said. "You have these two beautiful daughters, and you've spent the last four months whining about how you weren't able to give birth to Ada the natural way, and how maybe that was evolution telling you you weren't supposed to be a mother, and you did it right in front of me. I know it didn't make the home page of the Pasadena Star, but I cannot have children." Amy stopped for a deep breath. "I can't give birth naturally, I can't give birth through a C-section. Maybe you think that evolution is trying to tell you something, well, evolution is screaming in my ear with a megaphone you can't do this. You know how upset I was when I found out. Do you honestly think that I'm just going to...to be magically okay?"
"Oh Ames," Penny said, "I'm so sorry. I didn't mean to sound insensitive. Do you want to go out somewhere to eat? Leonard can watch the kids. I won't even talk about them at all."
Amy shook her head. "I'm busy tonight."
"Tomorrow, then?"
"I'm busy then, too." Amy reached for 4A's doorknob.
"Busy doing what?" Penny asked.
"I'll think of something."
"Amy, don't be like this."
Amy turned to face the blonde woman. "Maybe you weren't being selfish and insensitive on purpose," she said. "In fact, I'd bet money that you weren't intentionally trying to rub it in. But you did. I may have idolized you once," she acknowledged, "but now? Now I can't stand to look at you. I wish it was different. But it's not."
Ada was beginning to squirm, roused by the tense tone in Amy's voice. Penny rubbed her back absentmindedly. "Amy, I'm sorry. I'm so sorry. I love you, I don't want you to resent me."
Amy sighed. "I don't want to resent you either. But I also don't want to be infertile. Some things you just can't help." She turned and slipped into the apartment, closing the door behind her.
Penny stared at the closed door for a moment, then, her head shaking slightly, she stepped backward, and then backward again, until she felt herself hit something solid. Leaning against it, she clutched her daughter, sliding down to a sitting position on the floor, tears freely falling. I can't stand to look at you.
The last time Penny had hurt like this, Amy had been there to comfort her. Amy had always been there for her. Now, without realizing it, certainly without trying to, Penny had pushed her so far away that the scientist didn't even want to see her.
She nearly lost her balance when the door she was leaning on opened, and then she heard Raj's voice. "Penny?"
Penny looked up, craning her neck around to see the astrophysicist, looking down at her with concern. "Raj," she managed. "Amy hates me. Amy hates me."
Raj bent, putting his hands around Penny's upper arms and drawing her to her feet. He slid his arm around her and guided her to the couch, and when she sank down on it, he lowered himself beside her and, like she'd done for him several times before, drew her head to his shoulder and let her cry.
As we've never seen Amy get so furious with Penny in the show, this was both fun and difficult to write. Amy's been harboring these feelings for months. Penny's devastated, not good for someone who just finished therapy. Don't expect this damaged Bestieship to be repaired anytime soon.