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Solver was a robot that was sent to find the pieces of the Doomsday Chronometer.
They met Thomas Cromwell and stole part of his loot from him. They were commanded by The Clocksmith to help him in his work. The Eighth Doctor said that they were a legend on his planet. (AUDIO: The Eighth Piece)
When Risolva caught up with them she managed to get them under her control again and told them to disperse the clock throughout history. The Eighth Doctor learnt from Risolva that the rest of the Solvers had been enslaved by the Clocksmith, who had come to their world with trinkets which were used to disassemble her and he gained control over the rest of the Solvers. (AUDIO: The Doomsday Chronometer)