Alexander Martin was a Kanatian con man. He was also a trained medic. He knew Jack Harkness back when he was a con man.
He laid low on Earth for his retirement, living in Cardiff. (PROSE: The House That Jack Built)
He spent some time in St Helen's Hospital during the zombie attack on Cardiff. (PROSE: Bay of the Dead) A few days later, Jack Harkness hired him on a freelance basis as Torchwood Three's new medical officer. (PROSE: The House That Jack Built)