Here's a look at tonight's sci-fi TV:
Fringe, 'Over There, Part 1' – Leonard Nimoy is back as William Bell for the first half of Fringe's season two finale. Walter and Olivia try to cross over into the alternate universe to find Peter and prevent the collision of the two worlds. Orla Brady also returns as Elizabeth Bishop. (9 p.m. ET on Fox)
The Vampire Diaries, 'Founder's Day' – Expect lots of death and destruction as Johnathan Gilbert's plan brings chaos and terror to the Founder's Day celebration in tonight's season finale. (8 p.m. Et on The CW)
Supernatural, 'Swan Song' – It's another season finale as the Brothers Winchester come up with a plan to prevent the apocalypse and send Lucifer back to hell. (9 p.m. ET on the CW)
FlashForward, 'The Negotiation' – Everyone is waiting to see if their visions will come true on the eve of the flash-forward date. (8 p.m. ET on ABC)
Head after the jump for preview clips.