In this, Frisk isn't Toriel's daughter.

Sans had been looking for a book when they met.

"C'mon." He grunted, looking at the physics section. The skeleton couldn't see the book he wanted. "It's gotta to be here."

"Do you need help?"

Sans glanced at the end of the section, staring at the young woman. His cheekbones flushed blue as his sockets drifted over her.

She was dressed in a red sweater and grey skirt. Her brunette curls were pushed back by a faded red ribbon. Red glasses framed absolutely beautiful blue eyes.

He immediately felt guilt at that thought.

"Um, yeah. I'm looking for Quantum Physics by Eager Newton?"

She smiled, her smile sweet and dazzling. "Right here, actually." She said, pulling a cart into view. Pulling the book with its familiar green cover, she handed it to him.

As their fingers touched, Sans felt a static shock.

"Thank you, Miss..."

"Frisk. Frisk Mercy. I'm the new librarian."

He goes home, to eat a silent dinner with his wife. Sans and Toriel have been married for four years, yet their relationship has been falling apart before that.

He's pretty sure that they married out of sheer stubbornness.

As he's in the shower, Sans' thoughts drift to the librarian and the tiny peek of collarbone he saw. His magic rose up as he wondered what everything else looked like.

It gives him the best time of his life.

He comes back to the library, again and again, just to see Frisk.

She expects him by the fourth week. They talk as she finds him books. The human laughs when he jokes, throwing puns back at him.

Then Sans mentions his marriage, and her eyes sadden.

He tries to squash the feeling of hope.

Its after an argument when it starts.

Toriel had another late night at the school. Sans tries to ignore the hickies he sees on her throat. They argue about her 'work', and he ends up being sent to the couch again. He's lost track of how many times the couch is his bed now. As he lays there, listening to his wife get ready for bed, Sans wonders if Frisk would pull late nights.

When he hears Toriel go to bed, Sans feels his soul tug towards the librarian. He allows himself to teleport, hearing Frisk shriek in surprise. She sits on the couch with a book in hand. He allows himself to examine his surroundings.

He stood in the living room in what looked like a comfortable cottage. The walls are painted cream, the furniture either lavender or a candy red. A TV sits in a corner, turned off. A pot of flowers sits on the coffee table.

"Mr. Snowdin?"

His attention turns to Frisk, who had stood up. Concern was clear on her face. "A-Are you alright?"

"Yeah. I just wanted to see how you were doing."

She smiles. "I'm good. Just catching up on some reading." Frisk looks away, a flush on her cheeks. As Sans stares at her, he realizes that she's dressed in only plaid pajamas and this had entered dangerous territory.

He knows he should leave. Teleport back home.

To a cold couch. And a wife who has been sleeping with Asgore.

Instead, Sans takes a step forward. Lifting her chin, their eyes meet as he kisses her. Frisk kisses back after a moment, her arms drifting over his shoulders. She pulls away, shame and bright happiness mixing in her eyes. "W-We shouldn't. You're married."

"Please." he begs. The kiss starts up again, Sans fumbling to get his shoes off. Frisk pulls away again, this time to grip his hand. She leads him to her bedroom, shutting the door behind him.

It quickly becomes common.

Whenever Toriel has her late nights, Sans spends his breaks at the library. He learns all of Frisk's spots and he's pretty sure there isn't a spot in the library that hasn't been christened. The nights on the couch become nights in her bed. The late night trick is used.

Then Toriel announces she'll be gone for the entire weekend. "Some girlfriends want to go to this new mall a few hours away." she says as she packs.

More like Asgore.

Sans texts Frisk and invites her over once his wife pulls out of the driveway. Tori's gone the entire weekend.

Alright.

He opens the door when she knocks. His soul jumps happily at the sight of her, confirming his feelings.

He's falling in love with Frisk.

She sets the overnight bag in the bedroom, and the two's weekend begins. Besides sex, the two cook together and play games. Sans turns the radio on at one point, and they dance together.

A few weeks after their weekend together, Frisk looks sad. "What's wrong?" Sans asks, kissing the back of her head. They lie in bed together, both covered in sweat from three rounds. She turns, staring at his wedding ring. Envy is in her eyes.

"I'm tired of this."

At those words, his mind panics. What did I do?

"N-Not of you!" she says, reading the panic on his face. "I'm just tired of this."

"Of what?"

"Of being your mistress!" she explodes, sitting up. "Of sneaking around, of the lies and excuses. Do you know how many times my friends have tried to set me up with somebody since we start being together? I've used every lie in the book since I'm with you!" Tears start to fall and Sans is quick to sit up and start wiping them away. "I don't want to be your dirty little secret. I want to be with you in public."

"Frisk, Tori..."

"Talk to her." she pleads. "You told me yourself that you don't know why you're still together. Please, for my sake, talk."

He does.

When Toriel returns home, she starts in surprise to see Sans standing there. "We need to talk."

She nods.

They talk, agreeing that divorce is the best option.

A few weeks later, their divorce is finalized. Toriel moves out to live with Asgore, leaving Sans alone.

He publicly asks Frisk out a day or two later, and she agrees.