Cover Me is the sixteenth episode of the fifth season. It was released on 27 February 2009.
An old friend, who's a DEA agent, agrees to work with Liz in an undercover operation when Charlie comes up with a way to use math to affect the city's drug traffic. However, a miscalculation and an addiction endanger both Liz's life and the operation.
Possibly a reference to the Bruce Springsteen song, as well as its simple, literal meaning of mutual defense, and the 'deep cover' which is the requisite MO of the narcotics officer.