The relativity differentiator was a component inside TARDISes that allowed a TARDIS to travel in space. The TARDIS Sillarc had tried to steal from Gallifrey was undergoing repairs and had a damaged differentiator. While trying to pilot the TARDIS to his spaceship, Sillarc travelled millions of years into the past, but to exactly the same spot. After realising what had happened, he searched for a spare differentiator, when Plutar knocked him out and stole it. After being pursued by Sillarc, Plutar returned the TARDIS to his present and warned the Time Lords about Sillarc before he could get the differentiator back and take the TARDIS from Gallifrey. (COMIC: The Stolen TARDIS)

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