Smallville: Season 3, Episode 18

3 - 18: Truth

When Chloe accidentally inhales a mysterious krypto-gas, she discovers it acts as a truth serum to anyone who comes in contact with her and decides to take advantage of her new power by going to the Kents to find out Clark's secret. However, her new power comes with fatal consequences and Clark must find an antidote before she discovers the truth about him or worse, dies...