Angel Investigations' next mission finds them tracking a demon serial killer who preys on the city’s singles population. It's a burrower demon, a creature that possesses a host, finds its next prey then jumps to the new body - killing the previous host in the process.
During the investigation, Angel encounters undercover cop Kate Lockley. Kate, although sceptical of Angel’s vague credentials, becomes an essential source of information for the vampire. She’s pretty hot, too.
Trivia: The episode originally intended to follow City Of was called Corrupt. Although Lonely Heart features many of the same ideas, Corrupt was a much darker tale - particularly in its depiction of Kate. The network suits wanted a slighter fluffier show, however, and the script was reworked.