Mnemosine recall-wipe vapour was a substance used to induce amnesia or erase memories in a subject, like Torchwood Three's retcon pill or a memory worm; given its name, it was gaseous. River Song used it when she went to see incarnations of the Doctor which came before the first one she met (from her point of view) to not contaminate the timeline. (GAME: The Eternity Clock)

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