The second half of Season 5 has some real gems and "The Case of the Mystified Miner" is certainly one of them. This episode is based on the Gardner novel "The Case of the Spurious Spinster." The only misleading thing about that title is the implication that there is only one spinster.
"Mystified Miner" centers on Susan Fisher, a secretary for the Corning Company - owned by Amelia Corning. She works directly for Mr. Campbell, whose job is not particularly well described. Nonetheless, Campbell's son runs into the office on a Saturday and hands a shoe box to Susan (he says it's treasure that he needs to hide from the pirates, how cute). She discovers that it's filled with genuine treasure indeed - $200 thousand dollars! Before she can notify Mr Campbell, Mrs. Corning calls and tells Susan she's just arrived from South America and to pick her up at the airport right away. This sets off a chain of events that cause Susan Fisher to look awfully bad. Fisher (played by Kathie Browne, who was in 4 episodes of Perry Mason and 3 episodes of Ironside) does a good job playing the trusting but naive good girl. Every time she does something stupid, it's only a little annoying to watch.
The basic idea is that somebody is embezzling from the company, and the murder that takes place is undoubtedly tied into the embezzlement. Simple enough, but the relationships between the characters are unclear which can make the story rather confusing. On the other hand, the episode has excellent pace. From the moment the kid runs into the office with his "treasure," something interesting is always happening and it isn't until the very end that it actually makes sense. Like a lot of Perry Mason epilogues, there's a lot of explaining to do.
Della mentions her age to a mechanic (I can't tell if it's supposed to be a joke or not) and Tragg has a funny scene in Mason's office. There's also a clever tactic employed by Mason to tamper with evidence in a circuitous manner, which winds up being completely irrelevant. Overall, there are a lot of good reasons to watch this one.