When the Doctor was performing experiments on his TARDIS and got sent sideways to a parallel universe, Helen was affected too. She was trapped on 22 July at 11:15 am in a world where nothing moved except time and Helen. She worked out that she was travelling sideways, experiencing all versions of 22 July. She was able to affect small things near her, and she noticed that each day some things changed slightly, such as the government gradually moving toward the right and the text of novels changing punctuation from one day to the next.
Helen set up a routine for herself. She slept eight hours a day and went to UNIT headquarters every morning to see if the doors had opened. She wanted to leave a message for the Doctor, but nothing she could do was permanent.
After five years of living like this, Helen saw a man moving in the street. They moved toward each other, and Helen realised that she was moving sideways toward his reality, and he was moving sideways toward her reality. She figured that when they reached each other's worlds they would stop travelling. The thought gave her hope to get through the next five years. (PROSE: Still Lives)