This happens between Part 7 and Part 8 

One night Arja and Simoni were laying on the roof of their house, looking up at the stars, when Simoni pointed out a shooting star above them. Much to the girl's surprise the star started getting bigger, and suddenly both girls were bathed in a blue light as they were pulled up into a strange room made of gleaming chrome, both of them trapped in separate and seemingly indestructible tubes made from a glass-like material. This was their first encounter with the alien, Bethandi Shuda. 

An alien life form from some faraway galaxy, Bethandi's people came to Earth to study lifeforms from other planets. Her ship is a massive self-contained city and anything they abduct tends to wind up as an exhibit in an interstellar zoo of sorts that Bethandi's people maintain. Simoni went along with Bethandi's insistence that she allow them to examine her and collect some data on her species, after all she figured it was the only way back to Earth. Arja... did not. It took several researchers, a massive quantity of tranquilizers, and the near-destruction of one of their laboratories before they could get her under control. 

At first Bethandi had no intention of returning them to Earth (especially with Arja leaving five of her researchers needing medical care), but Simoni managed to negotiate a way out for them. Bethandi implanted a communicator/tracking device behind Simoni's right ear, the device looking like a small patch of silver skin right where her earlobe and head meet. If Simoni touches her fingertip to it she can communicate with Bethandi. The agreement was as follows: 

They could go back to Earth if: 
- They would find suitable replacements that the aliens could study. 
- They would be willing to come back to the ship on a temporary basis from time to time to serve as test subjects and zoo exhibits. 

After making this promise Bethandi did indeed beam the girls back to Earth, where Nelen and the others were all but ready to storm Castle Fullmoon to 'rescue' them, figuring that Franklin had finally captured the two. Thankfully they appeared right in the middle of their planning session and were able to explain that, while what had happened was bizarre as all holy hell, it wasn't Clan Fullmoon for once.