Popidius Celsinus was a member of the municipal council of Pompeii in 79 and an outspoken worshiper of the "Foreign Goddess" Isis. When he was a child, his father had paid for the construction of the Temple of Isis, which had allowed Popidius a place of honour and esteem in Pompeii.
He was interested in Melanie Bush, having been told that she and the Seventh Doctor were messengers sent by Isis, and that the TARDIS was a temple to her. His attempts to curry Mel's favour brought him into conflict with Eumachia, high priestess of the Roman god Triad, and spurned him into acting to free Mel when she had been sent to prison.
As the volcano erupted, he freed Mel and Aglae from prison, but Mel left the two of them to escape as she went to look for the TARDIS. On the way, they ran into the Doctor and Valeria, and when the Doctor went to look for Mel, the other three tried to escape from the city. (AUDIO: The Fires of Vulcan)