In a few of the early episodes of "Doc Martin", Ben Miller played Stewart James, a ranger who took care of the forests nearby. I know this because I read it...not because I could remember him or his character. So, although I've seen all the episodes, these never really stood out for me. And, after seeing "How Long Has This Been Going On?", I can see why I couldn't remember him.
Ranger Stewart returns and he wants the town thickie, Penhale, to help him work out his new survival course. Al Large also comes along, which in hindsight is a great thing, as something called Q Fever has made Stewart begin to hallucinate once they are in the woods. As you'd expect, Penhale is no use whatsoever. Ultimately, Doc Martin arrives to help untangle the mess.
An additional plot involvesa strange infection of the soil at the local school and its effect on the head mistress as well as many of the kids.
Overall, this is a very weak episode following a couple very good ones for season 10. The plots weren't all that interesting and Doc Martin seems like the only adult in the show. Skippable.