Smallville Season 7 Episode 6

Lara

Kara goes to Washington to look for her missing crystal, but is captured after she breaks into the lab. She's sedated with a truth serum that has kryptonite in it, which makes her relive a trip to Earth from the past. In this past trip she followed Lara, Clark's biological mother, to the Kent's farm. Clark saves Kara, but also ends up being affected by the serum when he burns it with his heat vision.