From season five through eight, Buddy Ebsen was featured less and less on his own show. I assume much of this was because of his age. But regardless why, having Jedediah and Betty helming most of the shows is very disappointing if you see the series. Neither has the personality of Ebsen but also it makes no sense to have Barnaby's secretary solving crimes...and Jedediah was too obsessed with his libido to make him likeable. Because of this, I was THRILLED when I saw "Design for Madness", as Ebsen was much more important to the episode....and actually did some reasonably strenuous work late in the episode.
The episode concerns Barnaby's niece*. She seems pretty normal when the show begins but soon she seems to have shot some guy with no provocation and she's lost her mind. Through the course of the show, Amy periodically does this...going from normal to goofy. However, as the episode progresses, Barnaby starts to wonder if maybe Amy is NOT insane but is being drugged with hallucinogens in order to get her permanently locked away in an institution. But who and why???
This is a very good episode and I thought the writing was better than usual. It did have a few plot elements that didn't seem totally thought out (such as why Amy was released soon after the shooting and Barnaby throwing out Amy's pills and testing her blood instead of just testing her pills). It also was a rather cruel episode considering what Amy goes through. But it IS interesting and is never dull. See the show...see what I mean.
*NEVER become a family member or friend of Barnaby, Jedediah or, especially, Betty. This is because whenever these folks are introduced, something BAD soon happens...like they are killed, accused of murder or actually commit a murder!