Chantir had been taken prisoner and was imprisoned in the same cell as the Doctor. With the Doctor's help, Chantir tricked the guards, escaped the prison facility and helped the Doctor recover the Key of Rassilon.
The Tenth Doctor stated that he could not remember Chantir because the De-mat Gun he used wiped most of his memories of the event. He had previously thought he had been alone in the cell. (COMIC: The Forgotten)