"Infinitum: Subject Unknown" follows Jane (Tori Butler-Hart) who wakes up in a strange attic, in a parallel world, with seemingly no way out. We soon realise that she is stuck in a time-loop, destined to relive the same day over and over with little memory of her doing it before. But with each 'reset' she starts to remember more and more and begins to piece together what may be happening to her, with clues pointing to a Professor Aaron Ostergaard (Conleth Hill).

​ She makes her way to the mysterious Wytness Quantum Research Centre, run by Charles Marland-White (Ian McKellen), to try to find a way out. As she travels from the city to the hidden research centre in the middle of nowhere, she sees that the world is entirely empty. There is evidence that life was once here, houses, cars and zeppelins hovering above, but everything is boarded up and abandoned.

Jane begins to get glimpses into other worlds and other versions of herself. At night, she sees torches and helicopters in the distance and discovers that soldiers are searching for her, adding to her desperation to get to the research centre and look for answers that might send her home. But each time she seems to be getting closer to an answer she is immediately reset back to the attic she fears, to once again begin her journey to learn the truth.

​ Once inside the centre the laws of time become erratic, Jane sees glimpses of people, scientists at the Research Centre from another time in another world. She begins to uncover the truth, that she is in fact stuck in an experiment with no way out.