In "Doctor Who", a version of River Song kills the Eleventh Doctor with what?

In "Doctor Who," River Song kills the Eleventh Doctor with poisoned lipstick. In the episode "Let's Kill Hitler," River—then still known as Melody Pond—has been conditioned by the Silence to assassinate the Doctor. She kisses him with lipstick containing a deadly toxin, and he begins to die within the hour.

This version of River is not a separate character but an earlier point in River Song's personal timeline. She is Amy Pond and Rory Williams's daughter, and she regenerates into River after being shot by Hitler. The episode reveals how the woman associated with the Doctor's future became his apparent killer.

The poison is especially dangerous because it is intended to prevent the Doctor from regenerating. The Eleventh Doctor survives only because River ultimately gives up her remaining regenerations to restore his life. A common mix-up is remembering River's weapon as a conventional poison or a gun; the memorable method is the poisoned kiss delivered through her lipstick.