Because Angela Lansbury was not exactly a Spring Chicken (in other words, she wasn't young any more), starting a few seasons into the series, they made a few episodes, here and there, where Lansbury introduces the show but she is NOT in the story itself. This was an attempt to give her a break from the rigors of filming...though it is odd that "Jack and Bill" came just after another bookend, "The Grand Old Lady". Unlike the two previous bookenders, "Jack and Bill" is more comedic...though all were apparently unsuccessful attempts to create spinoffs.
The hero of this story, Bill Boyle, is an ex-football player who is now a private eye...a BROKE private eye (Ken Howard). Unless something happens to change his luck, he simply cannot remain in business. When an old friend shows up, he (Max Baer Jr.) asks Bill to watch his doggy...just for a short time. However, when this friend ends up in the hospital with bullets in his chest, Bill is forced to keep the dog and try to work out the crime...with the help of the doggy!
The dog clearly was the star of the episode and how much you like it probably will have something to do with whether or not you like dogs in TV shows. I thought it was cute...some might think it is a bit dopey or schmaltzy due to the dog. Still, as a bookend episode, it's not bad and enjoyable.