This is rated as one of the best episodes of the series. But I'm stumped as to why, since it's basically just a clip show to bring new viewers up-to-date on the plot.
The setup is that the inevitable happens: David's hitchhiking lands him in the car of a dangerous nutcase. At least, that's the idea, but the hammy dialogue and acting makes the driver come off like a cartoon character. Anyway, one car crash and brief Hulk-out later, David has his face wrapped in bandages and a major case of amnesia. McGee shows up to question him about the Hulk's appearance, and they team up to find both the Hulk and David's (or rather, "John Doe"'s) identity. It's all makes a nice excuse for a half hour of Banner and McGee having flashbacks and discussions of the events of previous episodes.
There is stuff to like here: Jack Colvin's performance is excellent, and Bixby manages to get some compelling emotions across despite having to spend nearly the entire episode with bandages around his face. The premise is a good way to get the series's two main characters to actually interact for a while, and you wonder what's going to happen when an amnesiac Banner turns into the Hulk.
The trouble is that it all moves too slowly, stretching itself out with slow speech, unnecessary pauses, and lots of plot recapping of a series that only rarely bothers itself with ongoing plot line. It's dull and plodding, and ends on a cliffhanger without answering the questions it raised. In short, "Mystery Man" doesn't deliver a full episode's worth - or even half an episode's worth.