Namgoong Min transformed in order to be Han Ji-hyuk, and I have really little complaints with regard to his acting (and Kim Ji-eun was pleasant here, as well). The production and the action scenes are, like other have mentioned, excellent, but the plot remained too unfocused for it to be enjoyable.

I'll try to piece together the series through my own recollection, so if there's anything to correct or address, please tell me. About one year back, Ji-hyuk Dong-wook and Kyung-sook were tasked to encourage a North Korean high-ranking official to defect to South Korea. Because this would be good for the Foreign Bureau, though, I think Commissioner Lee In-hwan tapped Baek Mo-sa (because he's the fence in NK-China transactions) to assassinate that official. There were three people in the conversation: Baek Mo-sa, Chang Chun-woo, and the NK official.

Ji-hyuk's informer (who trusted Ji-hyuk until his death) told him that someone from his agency was informing on the three, and it was between his colleagues. Kyung-seok was the informant for the Domestic Bureau/Sangmuhoe, but before he could even kill the NK official, he was already dead. Dong-wook was assigned by the Foreign Bureau Chief Do to catch the informant, because Ji-hyuk was unwilling to rat or kill his colleagues. Baek Mo-sa's plan was to attack the dignity of the NIS, which was why he trapped Ji-hyuk in the room with the NK official. Because of his quick thinking and ability, however, he was able to escape and go to their team's meeting place.

Dong-wook figures out it was Kyung-seok, and kills him, and before Chang Chun-woo could clean up the mess Dong-wook shoots at him to protect Ji-hyuk. (They were, again, compromised, and I think Chang Chun-woo was Kyung-seok's cleanup crew in case shit happened.) However, Dong-wook gets shot at by Ji-hyuk because he can't tell him that his wife had just given birth and the gun was aimed at Ji-hyuk. (I think this was the reason he was imprisoned: involuntary manslaughter gives you 3-5 years in prison.)

Ji-hyuk escapes the bloodbath and has a nervous breakdown. He taps Chief Ha, because in an operation five years ago, he caught him embezzling funds from the agency. Chief Ha brings him the memory-erasing drug ZIP, and he purges his memory but sets himself on a quest to dig out the mole within the agency.

When he returns, he can't trust anybody and struggles to unearth the leads. Although he was able to reach his previous informant, the informant was killed by Sangmuhoe (Commissioner Lee wanted to cut off ties to everyone connected with the incident of the previous year). Eventually, because Su Yeon wanted out of the Domestic Bureau's shenanigans, she was also killed. That's probably why when she was being choked by Ji-hyuk, she also didn't resist, because she was also complicit in their team's destruction.

Ji-hyuk eventually figures out that it wasn't Chang Chun-woo who wanted Su-yeon killed because he loved her, and it also wasn't Director Kang, because he was previously in a relationship with her. However (and I think this is why he was imprisoned), he joined hands with Commissioner Lee, so he aided and abetted Sangmuhoe. He broke free after her death and tried to help Ji-hyuk afterward, especially after being abducted by Commissioner Lee (with a staged suicide attempt broken apart by Ji-hyuk).

Chang Chun-woo investigated the source of Sangmuhoe, which was actually a branch of the NIS. However, before they could interrogate Chun Pyung-il (which they discovered through the QR code), he was killed by Baek Mo-sa. So why does Baek Mo-sa not kill Ji-hyuk?

This is later on explained in the final episode, where he wants Ji-hyuk to be outraged and destroy the NIS from within. However, because of Je-yi's trust (despite the fact that he had her phone tapped and all), he was able to just follow through an avoid such a disaster. I mean, Je-yi also saw how he reacted around people whom he cared for or helped him (including her when he was angrily rampaging around chasing Chang Chun-woo to save her), so she knew he wasn't a monster despite everything and trusted him.

We later discover that Commissioner Lee has a stranglehold on public information through his collaboration with Planet, one of the largest social media providers in South Korea. That was why he could have anyone killed easily.

When the two of them finally linked Commissioner Lee with Baek Mo-sa and Sangmuhoe through the PDAs, the Executive Director planned to throw Lee In-hwan under the bus by using Commissioner Lee's next-in-line to give info to Baek Mo-sa that he was imprisoned. He wasn't killed, however, because Ji-hyuk came in time after his next-in-line transferred to a different country.

We later on realize that Commissioner Do didn't have anything to do with the botched operation that left Baek Mo-sa stranded, but it was Lee In-hwan's power play. Still, because everyone thought he came for revenge, they set up the plot during the memorial of Do's son, which was just a ruse.

The real plan (foreshadowed in the previous episode) was the use of an EMP to destabilize South Korea's largest bank, and give the NIS the same choice that they had to make with Baek Mo-sa. Ultimately, Baek Mo-sa's goal was to put the entire NIS in question with his planned murder of 33 people. (Right, before I forget, this was likely the item in question that Ji-hyuk obtained from the old drug-maker, but they didn't build up on this.)

Despite being shot at by her father, however, Je-yi trusts that a part of his is still within his dissociated identity, and takes this risk even though Ji-hyuk was unwilling. So in an act of sacrifice, she places herself with the hostages and hoped that part of her father would come back, which does. Ji-hyuk chooses to trust Baek Mo-sa's words and presses the kill switch before he's shot.

Ji-hyuk confesses to having killed one of his colleagues and suggests an investigation to Sangmuhoe. The encryption code will release the data that Planet's CEO gave to the reporter Jung Ki-sun who was deepfaked into being a North Korean spy, which would allow the NIS to weed out suspect agents.

Director Kang pays for working with Commissioner Lee by being imprisoned. Chief Ha and Commissioner Do were actually on the side of the NIS, and Chief Ha ends up as director and Yoo Je-yi as the manager. When Ji-hyuk gets released after five years, Je-yi remains his ever loyal fangirl and they share a silent moment before he gets back to work.

I'm glad I wrote this, because the plot also made more sense for me once I thought of it. It's still the weakest part of the series, and I won't recommend the series for those with low attention spans, but, if this wasn't clear, the real rat within was Commissioner Lee, who unleashed Baek Mo-sa.