Frisk was visibly uncomfortable at the idea of lying to Toriel. She knew it had to be done, to keep her from worrying about this new threat, but it still left her with a pain in her heart. She had never lied to Tori before. There was never a reason to not tell her the truth, as the old queen understood anything to the best of her abilities. 

But this situation was different. Frisk swallowed and nodded stiffly, keeping her mouth shut as they teleported away. 

It didn't take long for Toriel to open the door. She recognized Sans' knock well, and at first greeted them with delighted surprise. 

"Back so soon, you two?" She eyed them both. Sans' smile was fake, though it was hard for even her to tell. And Frisk was visibly shaking. The old goat's smile fell into an expression of great concern.

"H-Hey, Mom. We thought  we'd...Stop by for another visit. Is that okay?" Frisk did her best to seem cheery and happy, despite not moving from Sans' side. She didn't rush into Tori for a hug, and that was perhaps the most telling thing of all.

"My child, are you all right?" Instantly, Tori was over Frisk. She ran a paw through her messy hair, smoothing it and picking out some dirt that had been stuck in it. "What happened?" She turned her attention to Sans and put her other paw on his shoulder, giving him an equally concerned look. He looked worse than Frisk!

"It's okay, Mom! Really! It was just, uhh...A long night." Frisk stole a quick glance at Sans. "Let's just head in," she said as she took her mother's paw and held it. 

Toriel gave them a skeptical look, but stood up straight and moved to allow them inside. She turned her paw to hold Frisk's hand tightly, then led them into the hall.

Once inside her living room, Toriel fused over Frisk like she was mortally wounded. She got her a change of clothes, as her outfit had been torn and stained in the fight. It must have been when she was dragged around by those strings. Frisk, of course, didn't dare say that. She simply did as Toriel told her. After that, the old goat sat her down on the couch and gently cleaned the dirt from her face. She also bandaged a small scrape that was on Frisk's cheek.

"Mom, really! I'm okay, I promise!" In all honesty, Frisk was a little embarrassed. First she couldn't do much to help Sans in the fight, and now Toriel was completely babying her. She appreciated the concern, but didn't feel like she deserved it. 

All the while, Toriel was quiet. Every once in a while,  she'd glance at Sans, as though waiting for the real reason they were both in such a state. She didn't pry it from him, though, and simply let them both relax and eat some of the leftover pie.

As soon as Sans was gone, however, Toriel stood firm and crossed her arms. "Frisk, I want you to tell me what really happened. And I want you to tell me now." 
Frisk stared in the direction Sans had been, startled by Tori's sudden firmness. She looked up at her, feeling like a small child that was dangerously close to being grounded. "I...I did tell you! Nothing happened, it was just--"

"A rough night. Yes, you have said that many times now, my child. No matter how many times you say it, I will not believe it."

Frisk slouched and looked to the ground.

"Tell me the truth."

"I...I can't, Mom. I'm sorry, but I can't."

"Why not?"

"Because you won't understand!" Frisk shot her head up to look Toriel in the eye. From her spot on the couch, it was hard to see her expression clearly...But she knew it was a hurt one. "I mean...It's complicated. Please, just trust me when I say it's for the best that you don't know. That no one knows."

Toriel let her arms fall to her sides and stepped back. She turned away from Frisk. "I see..."

Panicking, Frisk stood and went to Tori. "I love you, Mom. It's nothing that's worth worrying about, I promise. Everything will be fine!"

"You're not fine now, Frisk!" Toriel rounded on the young woman, her expression fierce with hurt and worry. She gasped when she realized she had just yelled, and lifted a paw to her chest. "Please," she said much more softly. "Tell me, my child."

Frisk stared at her for a long moment, then lowered her head. "I can't...I'm sorry." Her bottom lip quivered, and she quickly bit onto it to keep from crying. She had shed far too many tears that day.

Toriel was quiet. She waited a moment, then slowly nodded. Again, she turned away. "I am sorry, my child, but I do not want you coming to me in such a condition again. My soul cannot take it."

"I understand, Mom." Frisk slowly stepped up to her and put a hand on her arm. "You don't have to worry about me, I promise."

Toriel didn't respond. They stood like that for a long moment before Frisk finally stepped back. "I'll...wait for Sans in the hall." She paused, and when she didn't hear any argument, she slowly headed towards the stairs. She could feel her heart shatter from the immense guilt, and it was not unlike all those years ago when Toriel begged her not to leave the ruins. 

Downstairs, Frisk leaned heavily against the door. She tilted her head up to look at the ceiling, trying to get the tight, sinking feeling in her chest to go away. This was for the best...She had to tell herself that. Sans was right - no one could know about that other Sans, or the possibility of other realities. They had to stay blissfully ignorant.

So why did she feel so bad? Sucking in a breath of the cold air, Frisk put her face in her hands. She took slow breaths as she waited for Sans.