[s1e15] Masquerade -

Damon traps Katherine in the tomb, where she warns him that Elena is in danger because she is the "doppelganger". The episode ends with Elena being kidnapped by a masked figure. Season 1, Episode 15: " A Few Good Men "

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. [S1E15] Masquerade

Alaric attempts to kill Damon but is killed himself. He remarkably comes back to life, revealing for the first time the power of his protective Gilbert Ring. Damon traps Katherine in the tomb, where she

This episode is a high-stakes turning point where the main cast finally coordinates to take down Katherine Pierce. She breaks the link and hands the moonstone

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

Elena discovers that her birth mother, Isobel, was Alaric Saltzman's wife and was turned into a vampire by Damon.

Damon participates in a fundraising auction, using the platform to taunt Alaric about his "delicious" wife.