Virginia McKeller (Pamela Franklin) hires Mannix to help her find her sister, Jennifer and stop her from messing with her things. In other words, she thinks Jennifer is sneaking in to her home and taking or re-arranging things. This sounds pretty crazy...but is even crazier when Cannon finds out that Jennifer died long ago! Through the course of the case, he also finds another 'sister'....Cyndi. Cyndi is very different from Virginia...but all three are actually the same person as Virginia is mentally ill and has Multiple Personality Disorder (now called Dissociative Identity Disorder).
The whole multiple personality angle is just silly and makes this a very hard episode to endure. It's a real shame, but back in the 70s this was a big issue (with films like "Sybil") but most therapists today feel the problem is extremely rare, as most cases have since been proved to be hoaxes. Of course the "Cannon" folks couldn't know that but the episode is still silly and sloppy and is filled with mumbo jumbo.