D.P. stands for "Deserter Pursuit". It's the branch of the South Korean military police whose mission it is to track down deserters. These are people who are doing their mandatory military service, hunting down other people who are fleeing their mandatory military service. The Wikipedia sypnopsis for series 1 says:
Private Ahn Jun-ho and Corporal Han Ho-yeol team up to find the deserters, and they end up on an adventurous journey.
And at first you might think it's about adventure - there's a degree of comedy involved in the odd couple relationship of those 2 very different characters. Jun-ho is serious and taciturn. Ho-yeol is garrulous and a bit of a chancer. Together they form an effective team.
However, although the series has comedic moments, it's also... How can I put this? Really, really dark. You're always a brief moment away from tragedy and outright horror. The horror - and much of the tragedy - comes from the slowly building, overarching storyline throughout the series, about bullying in the military, and how this dehumanises otherwise good or kind people, sometimes to the point of breaking them completely.
The bullying itself is horrid to see depicted on screen - sometimes it's of a sexual nature, sometimes it's people being beaten up, always it's people being treated in a derogatory fashion. Some of the individuals who are bullied are driven to fleeing their posts, and then Jun-ho and Ho-yeol are appointed to track them down.
But it's not as if Jun-ho or Ho-yeol are unaware of what goes on either. They're also subject to some bullying or targetting by higher ranks, and they witness their peers being maltreated too. Without giving spoilers away, when things hit the fan in a big way in the first season, they have to sit with the knowledge that they also didn't do anything to try to stop the bullying. At the same time it's clear that the problem is systemic. Even their supervisors of higher rank also recall experiences of being harrassed, beaten up, etc, when they were of lower rank.
I understand that this show caused a lot of discussion in South Korea when it first came out, with various men speaking up about having similar experiences during their compulsory military service. So it's obviously not based on nothing. There's a kind of searing social truth being explicated in the drama of this show. It's impossible to watch it without feeling critical about the institution, but also especially about the higher ups who try to conceal wrongdoing, and enable bullying to persist as part of the overall culture that young men are being funneled into.
South Korean films and TV shows are often good at being critical about the wealthy and powerful, and D.P. is similar in how it portrays the behaviour of individuals with status, and who seek only to protect themselves within the system, at the expense of the innocent and less powerful.
Would I recommend this series? I'm about to start series 2, so I guess so. But not without content warnings for all the things I've mentioned above, plus some other details that I can't go into without giving spoilers away. Basically, it's very tough to watch when it depicts abuse, but I think it also does a good job of showing how corrosive that is as an institutionalised phenomenon. And hey, I come from a different country that also has mandatory military service for young men, and let's just say I have personally heard some stories too. This is a TV series about a particular country and culture, and a specific institution, but at the same time some of this can be generalised out into many structures where some people are accorded status or power and then misuse that to the detriment of everyone else.