Beth Stokes was the leader of a Valkyrie unit. When she led her unit to a Scientific Facility Rawke 279, she wished it to be evacuated after her crew were treated for injuries by the Sistermatic. She blamed herself when the Daleks appeared to have followed her. This drove her mad, to the point where she deactivated the defence grid to let the Daleks in.
Her colony had been invaded in her youth. Her older sister Becka and she had been taken into a Dalek work camp, where Becka died. Beth's mind returned to her when Ace revealed that it wasn't her fault for the Daleks being there. She chose to stay behind so the Black Dalek's self-destruction wasn't tampered with by the Kiseibya. (AUDIO: Enemy of the Daleks)