Sarah Jane is thrilled when she successfully bids for a Victorian brass head in an antiques auction. On taking the metal model home, Mr Smith informs her that it contains a research probe from the future.

Sarah Jane speculates that the probe was used as the 'brain' that powered the 'Difference Golem'; a mechanical servant built by eccentric inventor Sir Joseph Montague. She suggests destroying it, to prevent such advanced technology from falling into the wrong hands, but Rani persuades her that this would be wrong, as the golem could have been sentient. So she, Rani, Sky and Clyde travel to Holcote House, the former residence of Sir Joseph, in search of its body. But they are soon to discover that meddling with artificial intelligence can have dangerous consequences...