In On The Game - S11.E40

Sgt. Boyden and probationer PC Nick Slater are called to the house of an elderly lady who claims a couple are fornicating in her garden. A search of the surrounding area turns up the guilty parties: a wealthy local businessman and his companion, Lisa Royle. Drugs are also found in their car, so the two are taken into custody. Lisa flirts with Boyden, and gives him a phone number to call to vouch for her identity. She turns out to be DS Lisa Holm on an undercover operation, and an angry DI Raymond turns up at Sun Hill to ensure the operation is not further compromised. Slater is confused when Boyden suddenly releases the pair and demands to know why, but Boyden tells him that as a probationer, there are a lot of things he doesn't need to know.