[s1e15] Masquerade «Tested & Working»

Lucy reveals herself as a distant relative of Bonnie Bennett and turns on Katherine once she realizes Bonnie is "one of the good ones". She breaks the link and hands the moonstone to Katherine, but also incapacitates her.

If you specifically meant S1E15, this episode focuses on the mystery of Elena's birth mother and the fallout of the tomb opening. [S1E15] Masquerade

Elena discovers that her birth mother, Isobel, was Alaric Saltzman's wife and was turned into a vampire by Damon. Lucy reveals herself as a distant relative of