A girl is mysteriously transported into an alternate reality, some kind of parallel universe. She recognizes structures from her world, but here they are ruins. This is a world of magic, not science. Here the laws of magic govern things. She recognizes people she knew in her home universe. Except here, they are different. Here, she might meet someone who was her friend who ends up being a hostile enemy. And...the things she does in each universe has an impact on the other.
In the magic universe, she finds that she was summoned by a wizard. He gives her a magic crystal and sends her back. He tells her she must find his counterpart in her universe. When she finds him, he’s a brilliant scientist building a super collider in his garage. It requires a lens to make it work. Rather than using miles and miles of structure, he only needs a span of about 6 feet. But it is purely theoretical. To make it work, he needs a crystal structure beyond the capabilities of modern technology to produce. But...not beyond the means of magic! The crystal that the girl brings is just such a crystal, perfectly shaped and cut to make his device work! Thus, securing the portal between the 2 worlds. He (both scientist and wizard versions) then serve as her entry way into each world, and her guide. They each pose explanations (one using science and one using magic) as to what is happening (they help the audience understand wtf is going on.)
She must maneuver both worlds trying to save her own, all the while making sure not to alter either too much as it could have unknown repercussions that would ripple like a domino effect through both, potentially destroying them both.
This would make a great series. Her quest to save her world could span multiple seasons. Each episode would involve a problem, and a series of solutions that would initially make things worse as the implications of her actions in one universe ripples into the other. Eventually, a final confrontation would have to occur, thus ending the initial story line. Whether this is an end to a single season or an end to the series would have to be determined. The story could be stretched over multiple seasons. Or, upon saving her world and Thera, perhaps another rift opens and she now finds herself navigating a completely different universe once again!
If enough material can't be invented to maintain multiple seasons, this could also be tightened up and work well as a feature film also.