Smallville Season 10 Episode 10 (12/03/10)
Tess inherited a Kryptonian artifact from Lionel which Clark activated, swapping him out with his doppelganger from Earth-Two, an alternate world, where Clark was found and raised by Lionel Luthor, is a murderer going by the name Ultraman, killed Lex, and is plotting with his lover Tess to overthrow Lionel.
On Earth-One, E-Two Clark arrived with a thirst for blood, happy to be in a world where Lionel wasn't there to control him. Tess played along with his belief that she was his lover long enough to contact Lois for help. E-Two Clark heard their conversation in the Watchtower and attacked them.
On Earth-Two, Lois and Oliver are engaged and they hate Clark. Lionel set E-One Clark up by telling him Oliver had the box. Clark kidnapped Lois and ransomed her for the box. When he met Oliver at the Watchtower, Oliver trapped him with kryptonite and was going to kill him, until Lionel intervened, only to turn and try to kill Clark himself. Oliver heard the conversation and disabled the kryptonite and Clark was able to use the box to travel home to Earth-One where Tess, Oliver and Lois were in the throws of a battle with E-Two Clark.
Just prior to his transfer to Earth-Two, Clark was ranting to Tess about how Luthor blood is poisonous, unaware she was Lionel's sole living heir. Before she could tell him, he vanished.
Upon his return, he apologized to Tess who confided that she was sad Lionel abandoned her as a child, as sick as that was, and that hiding Lex's clone was a desperate attempt to have a family. Clark promised her she wasn't alone anymore.
Earth-Two Lionel showed up in Metropolis on Earth-One.