Serenity was a feral cat in England in the aftermath of the 22nd century Dalek invasion. Becca, a young girl, attempted to domesticate her. Her name was ironic, because her demeanour was anything but serene.
In Becca's eyes, the cat was crucial to her family, because it could produce enough offspring to adequately patrol their farm for rats. In the wake of the Dalek devastation of England, pest animals, which normally would have been controlled, were running rampant. This meant that rats were threatening to eat Becca's family's supply of grain.
Because pest control services had been interrupted by the Daleks, the price of cats had risen astronomically. It was therefore critical that Becca be able to breed the wild Serenity to secure her family's grain stocks.
This proved difficult, since Serenity would not stay on the farm. Indeed, Becca had to chase the cat frequently, which put the girl into serious jeopardy on several occasions. On one such occasion, she was saved from an unexpected encounter with a slyther by the fortunate arrival of a knight named Donna. The two bonded over the wild cat.
Later, Donna returned to Becca's farm with the Eighth Doctor. Becca presented the Doctor with a tabby from Serenity's litter, who travelled in the Doctor's TARDIS for an unknown period of time thereafter. (PROSE: Legacy of the Daleks)