I do not own Doctor Who or Once Upon a Time. I only own this story. Enjoy.
Chapter 1: Ariel's Mission
On a beautiful day in Cardiff, the sun was shining brightly, people were peacefully walking the streets, and general happiness filled the air. The atmosphere of the city was practically magical. However, outside the city on the shores of a quiet beach, a red haired mermaid swam around with evil intentions for the city and it's protectors against aliens, magic, and the supernatural: Torchwood. Her name was Ariel and she was on a mission from Peter Pan, the most malicious boy in the universe.
Ariel first caught Pan's attention when he saw her dive through the waters to save Prince Eric, a human whom she loved. Pan decided to use this love against her. Peter has a devious plan in mind that requires the two most powerful objects in existence, the red diamond and the blue diamond. Both are small jewels that each contain unspeakable power. For example, one could use either diamond to destroy an entire planet or control millions of people at once. Years ago, Rumplestiltskin kept these two diamonds in his castle. However, he knew that all magic comes with a price. Since this magic had extraordinary power, he was sure that the price would be too much and therefore rarely ever used them.
One day, the Evil Queen was able to win the red diamond from him. To make sure that she would never get her hands on the blue diamond, he used magic to send it to a parallel world, and he almost destroyed the universe in doing so. The Evil Queen knew that an agency called Torchwood had once built a device that allowed someone to travel from one universe to the other. So, she recruited Prince Eric and his crew of skilled soldiers and went through a portal across the stars from the Enchanted Forest to Earth, bringing the red diamond with her.
However, Regina soon found that Earth was a land without magic. Only when the Cardiff Rift was open could she use her special powers. Despite this, the red diamond's magic was still active, even when the rift was closed, because of its immense power. When Regina discovered that Torchwood could not help her, she almost destroyed the planet. The Doctor and Clara had to come and help stop her. She then retreated back to the Enchanted Forest, taking the heart of the newest Torchwood agent Mickey Smith with her. Now, Jack Harkness, Gwen Cooper, Ianto Jones, Martha Jones, and Mickey Smith keep the red diamond locked up safe where no one can get access to its power.
However, if Pan's ultimate goal was to be achieved, he needed to get the red diamond, which is where Ariel comes in. Mermaids have the ability to travel between different worlds and different universes. Pan told Ariel to go to a certain parallel world and retrieve the blue diamond for him. He threatened to kill Prince Eric if she didn't. He gave her a special bracelet that when worn, magically turns her tail into two human legs. Before Ariel even met Pan, her voice was stolen by the Evil Queen, as revenge for aiding Snow White. Fortunately for Ariel, she met the Doctor and Clara Oswald. The Doctor gave her a telepathy pendant that he had picked up during his many years of traveling the universe. The special pendant allows her to telepathically insert her thoughts into other peoples' minds. It's not exactly talking, but it's the next best thing. Ariel used the pendant when she went to the parallel world. What Ariel did not know, was that this world was infested with Cybermen.
Ariel met Rose Tyler, a longtime enemy of Pete Pan, and the Human Tenth Doctor. They helped her retrieve the blue diamond. Ariel witnessed the diamond's power when she saw Rose use it to destroy every Cybermen on that parallel world in a matter of seconds. Rose then expressed an interest to go back to her native universe with Ariel, so she could help stop Pan from achieving his evil desires and hopefully find the Time Lord Doctor again to get the closure on their relationship she needed in order to further pursue her relationship with the Human Doctor.
When they returned, Rose met up with Jack Harkness, who escorted her back to Torchwood where she could sleep until the next morning. Ariel then met up with Pan and gave him the blue diamond. Pan then told her to get the red diamond from Torchwood by any means necessary, or Prince Eric's life would end. Ariel now wears her enchanted bracelet and walks down the streets of Cardiff. She is determined to do whatever it takes to get the red diamond and stop Pan from killing Eric. Ariel knew that to save his life, she could do anything. She'd even kill all of Torchwood and Rose Tyler if she had to.