When one of the team members, Frank Paxton, and his fellow terrorists attacked the rest of the team, Catherine jumped into the cyclotron to escape him. In the cyclotron, she relived moments of her life, able to choose which events she experienced but unable to change them.
The Fifth Doctor, on arriving at the base and realising someone was trapped in the cyclotron, jumped in to retrieve her. He convinced her to leave the cyclotron and brought her back to Earth prior to the time she left for Hardy. (PROSE: Lackaday Express)