Transformers: Animated

Along Came a Spider (1x9)

:

It's Halloween and Sari introduces the Autobots to Earth's costume-and-candy traditions. However, the entire affair brings up bad memories for Optimus, who is haunted by the ghosts of his past, specifically when he and his fellow bots, the cocky and arrogant Sentinel Prime and power-mimicking Elita One were looking for a crashed Decepticon ship loaded with energon cubes. When Sari takes Bumblebee and Bulkhead trick or treating, the spider-like Decepticon Blackarachnia shows up and targets Sari's key. She wants the key to get rid of her organic half that she got when she was on an organic planet infested with giant spiders. The spider's venom somehow mutated her Cybertronian systems, making her 'a freak'. After seeing how Sari's key can heal Autobots, she wants to use it to purge her organic half. However, her body can't survive without her organic half now and as a result of using the key, she nearly kills off all organic matter in the entire city (which includes herself and Sari). Optimus gets the key away from her and Blackarachnia escapes, alone once again.