Molly smiled as she pet her dog, Krypto. Her parents had recently gotten the dog, a white mutt, for her tenth birthday. She kept begging for a dog of her own, especially one that reminded her of her favorite cartoon dog, Krypto the Super Dog. Her parents had finally found a dog that was perfect for her, and surprised her.
"Guess what, Krypto?" Molly said, ruffling his fur, "We get to stay home alone today!" 
Her parents were both working late, leaving the girl alone for at least three hours that evening. They had decided she was finally old enough to be trusted, as long as she had a dog to protect her.
"Now, please behave. Dad will be back first. If you need us, our numbers are on the fridge," Molly's mom said, heading out the front door.
Molly nodded and listened to the door lock, then her moms car drive away. 
"What should we do first, boy?" Molly asked.
Krypto walked to the back door and whined, asking to go outside. Molly obediently opened the door and let her dog outside, following him. She had been warned multiple times to not allow him out alone, since he hadn't been neutered yet. Her parents didn't want him getting any neighborhood dogs pregnant. Krypto sniffed around the yard for a while, marking a few places as his, while Molly sat back in a chair on the deck.
Glancing around to make sure no one was outside, Molly slipped off her shorts and t-shirt. She had recently been learning to explore herself after sex ed had begun being taught at school. It had gotten her curious about what it all was like, but hadn't had much opportunity to try anything since her parents were always home, or a babysitter was with her.  
Molly slid off her panties, a stereotypical pair that was pink with little flowers. Her mom had picked them out for her, and she wasn't a fan of them. Gently, she began to rub over her clit, moaning softly as she did. It felt amazing to finally have a chance to explore openly and without fear. Already she was getting wet. She slipped her finger lower to her slit and teased it. Her eyes closed as she slid her finger into her wet pink pussy for the first time ever. She gasped as she felt herself, surprised by how warm and tight her hole was.
Getting more into it, Molly began to move her finger in and out faster, considering adding a second finger, when something surprised her. Something warm and wet ran over her slit and clit. Eyes opening wide, Molly yelped and sat up. Krypto stood in front of her, tail wagging, tongue hanging out of his mouth. He tilted his head, wondering if he could get another taste of what his human had to offer.
Molly stared at her dog, remembering in class that they had discussed oral sex, and began to wonder if it was possible to do it with a dog. She removed her finger from her hole and spread her legs wider, settling back into the chair again. 
"Come here, boy," Molly said, coaxing her dog gently, "It's okay. You just surprised me."
Krypto moved closer to her, sniffing cautiously. What he smelled was sweet and intriguing, tempting him to go for another lick. He slid his tongue over Molly's wet slit, ears perking as he heard Molly moan.
"Good boy," Molly moaned, loving how soft the tongue was.
Krypto knew those two words well, and began to lick more quickly, finding that the more he licked, the more of the sweet nectar he could get. Molly was soon writhing in pleasure as Krypto tried desperately to get his tongue deeper into her. Unbeknownst to the girl, her dog was getting just as excited as she was. His red dog cock was poking out of his sheath, swelling up and dripping. 
Molly began to feel a new sensation, a strange but tantalizing tingle in her pussy. Right as she felt she would explode, Krypto abruptly stopped his licking. 
"Krypto, why'd you-" Molly started to say, but quickly stopped when her dog jumped up and out his paws on either side of her. 
Molly giggled and hugged her dog, feeling him squirm and buck over her. She didn't realize that her dog had had enough licking and was ready to fuck. 
"What are you doing?" Molly giggled.
She gasped as she got her answer. Krypto shoved his thick red rocket into her virgin hole. Molly clung to her dog as he began to thrust his hips desperately, ready to make her his bitch. 
"K-Kyrpto! You're such a good boy!" Molly said.
Her dog humped her eagerly, his cock growing more. Molly was unaware, however, that dogs were different from humans. She had gotten to see what a human male cock looked like in class, so she didn't know that Krypto had a thick knot at the base of his cock.
Soon she could feel Krypto growing thicker inside her near the entrance to her pussy. His knot was growing. The strange tingling feeling was back for the girl as she got knotted by her dog for the first time. 
"Good boy, Krypto! Don't stop, please!" Molly moaned.
The feeling washed over her body, making her entire body shake. She was having her first orgasm. Her pussy tightened and squeezed around Krypto and his knot, pushing her dog over the edge too. He began to spurt his hot doggy cum into the humans pussy. Molly held onto him, panting and moaning.
She knew that she had to find an excuse to keep her dog from getting neutered so they could continue to have this new relationship together, and play their new game again and again.