Enemy of the Daleks
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Beth Stokes was the leader of a Valkyrie unit. When she led her unit to a Scientific Facility Roarke 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. She and her older sister Rebecca had been taken into a Dalek work camp, where Becka died. Beth regained control of her mind 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. (Enemy of the Daleks)