Postman Pat has to deliver Dr. Gilbertson's sister's cat, Bess, as part of the Special Delivery Service. Bess looks exactly the same as Jess, so when she escapes everyone in Pencaster and Greendale thinks that she is Jess. This is not good for Jess as the two have very different personalities.
Bess is unwilling to come down from a lamp post, which she has climbed up to escape Bonnie and has to be rescued by Pat in the SDS Helicopter.
In delivering Bess, there is a mix up in communication with Dr. Gilbertson, but she is eventually delivered in time.