Dancer was a pirate on the Fancy under the command of Henry Avery in the 17th century. He fought with Amy Pond and got his hand cut, giving him the black spot. He was then taken by the Siren to her ship. He later joined the rest of the crew when they took the ship for themselves. (TV: The Curse of the Black Spot)
Some time later, the Doctor recruited Avery to help him with an attack on the asteroid of Demon's Run to rescue Amy Pond. Dancer and the crew boarded the ship of Madame Kovarian during the attack and subdued its crew, preventing her escape. (TV: A Good Man Goes to War)