Perry Mason never explained the case of his disappearing legal aide David Gideon
The character was mysteriously cut after nine episodes.
Perry Mason perfected its formula to the point where its formulaic episodes actually became a sign of the show's strength for loyal fans. In the Perry Mason universe, there were lawyers, secretaries, judges and detectives, and anyone outside of those archetypes better be a killer or a victim or else they simply felt shoe-horned in, an unnecessary accessory in the case of every crime. That's the biggest criticism fans have for David Gideon, the handsome young student aide who showed up in season 4 to help and learn from his hero Perry Mason.
David Gideon was played by the actor Karl Held, who joined the Perry Mason cast for nine episodes across two seasons, but nobody's really sure exactly why. In most of his episodes, his character was kind of useless, occasionally making mistakes that Perry Mason had to work past. Some believe the idea was to introduce a young character to boost ratings. But if that was the plan, it backfired. His character largely divided fans instead of drawing more.
Perhaps the biggest flaw in the plan to work David Gideon into Perry Mason was that writers never gave him a compelling reason to exist. He wasn't an original character, and he didn't add meaningfully to the plots, apart from occasionally providing insights that it turns out Perry was, of course, already wise to. If Perry Mason was infallible, then David Gideon could add little as a sidekick that Della Street and Paul Drake didn't already have covered. In fact, some fans even grumble that Gideon stole scenes from Drake!
But if you look back over the David Gideon episodes, there is an arc to follow in the character. He begins in "The Case of the Grumbling Grandfather" as a bit of a fool who falls for a conniving woman's tricks. The boy is blindly noble, and it bites him by the episode's end. It also surges his admiration for Perry Mason, and as David Gideon begins to learn from the master, there was maybe a missed opportunity to explore that dynamic. In the end, it just was unfortunate that many fans felt Gideon's scenes took time away from the profound chemistry between the show's more established characters.
And that's how we come to one of Perry Mason's biggest unsolved cases: What ever happened to David Gideon? After a scant appearance in the fifth season episode "The Case of the Shapely Shadow," the ambitious law student just disappears. He's never mentioned again, and we never got a goodbye.
Classic TV fans, of course, saw Karl Held appear on many shows through the '80s, including 77 Sunset Strip, The Outer Limits, Star Trek, Charlie's Angels and The Jeffersons, as well as playing Garth on the '80s soap opera Falcon Crest. So while his Perry Mason character remains an unsolved mystery, at least we knew the actor escaped the show unscathed by his early character's icy reception.