I just watched finish it and one word: AWESOME

Kim Soohyun (who happens to be my favourite actor <3) is the lead of the show. HE WAS DAMN CUTE (and handsome). Lee Hyun Woo (who i happen to like to) is inside too! He is damn cute too!

Here is the Plot copied and paste from wikipedia:

A group of spies called the 5446 Corps were trained by North Korean elite special forces since their youth, ostensibly towards the lofty goal of unifying Korea. They are ambitiously dispatched to South Korea, where each disguises himself as a fool, an aspiring singer, and a high school student. Passing boring time without receiving any orders from the North, they gradually get used to life as ordinary neighbors in a small town until one day, their mundane lives are turned upside down when a "secret and great" mission is suddenly assigned to them. Due to the events of the Second Battle of YeonPyeong, South Korea demands the names, location, and rank of 30 North Korean spies active in South Korea, promising financial aid to the North on the condition that Pyongyang turns in their spies. To prevent the elites from falling into enemy hands, the North Korean government orders dozens of active duty spies to take their own lives. Meanwhile their North Korean army instructor Kim Tae-won crosses the border to eliminate those who refuse to follow.

If you are interested to know the cast, here it is:

Kim Soo-hyun - Lieutenant Won Ryu-hwan / Bang Dong-gu - North Korean top agent who beat out 20,000 other competitors, is fluent in five languages and has an uncanny ability to read people. Disguised as village idiot.

Park Ki-woong - Rhee Hae-rang / Kim Min-Su - Son of a high ranking North Korean official Rhee Moo-Hyuk, and as an agent is almost as good as Ryu-hwan. Disguised as a singer-wannabe.

Lee Hyun-woo - Rhee Hae-jin - The youngest secret agent in North Korean history. Disguised as a high school student.

Son Hyun-joo - Colonel Kim Tae-won - 5446 Corps training chief

Park Hye-sook - Jeon Soon-im - "Market granny"

Kim Sung-kyun - Seo Soo-hyuk - NIS team leader

Go Chang-seok - 2nd Lieutenant Seo Sang-gu / Professor Seo Young-guk

Jang Gwang - Go Hwi-sun - "Old man Go"

Shin Jung-geun - Mr. Park, the barber

Hong Kyung-in - Jo Doo-seok - Soon-im's son

Lee Chae-young - Ran(Nickname)/Heo Ih-ran(webtoon)/Heo Jeom-ran(film)

Park Eun-bin - Yoon Yoo-ran

Choi Woo-shik - Yoon Yoo-joon - Yoo-ran's Younger brother

Joo Hyun - Rhee Moo-hyuk - 5446 Corps founder, Hae-Rang's Father

Gu Seung-hyun - Hwang Chi-woong

Jo Yong-jin - Hwang Se-woong - Chi-woong's Younger brother

Lee Yeon-kyung - Chi-woong and Se-woong's mother

Uhm Tae-gu - Hwang Jae-oh

Moon Won-ju - Choi Wan-woo

Kim Beop-rae - NIS director

Lee Min - department head

Kim Young-jin - Kim Hee-kwan - former MPAF head

Yoon Won-seok - Deong-chi

Park Jeong-gi - strike-force team leader

Go In-beom - NIS chief Choi Jin-Tak

Lee Bo-hyeon - NIS agent

Yook Se-jin - NIS agent

Kang Eun-tak - NIS agent

Park Jang-shik - NIS agent

Won Hyeon-jun - NIS agent

Heo Seok - NIS agent

Son Jun-young - young Ryu-hwan

Hong Tae-ui - young Hae-rang

Seong Yu-bin - young Hae-jin

Kim Do-gyun - audition judge

Park Hwi-sun

Find out more at Wikipedia.

Watch the trailer here (english subtitles provided).

Watch the movie here (English subtitles provided) (Playlist 4 is recommended).

My opinions: 

Honestly, I loved the plot. North korean and south koreans at war, Secret troop sent to south korea as spys. I like how they disguised themselves. I think the actors potray their characters really well too especially Kim soohyun.

I don't know how to express my thoughts because it goes beyond what words can express. There are funny parts where you can laugh to, sad/touching parts where you can cry to It may not be the best movie ever but it certainly touches your heart and make you feel that it is the best movie at that moment. It even inspires me to write a fanfic about it :/ 

With handsome guys and good plot, this is a definitely a MUST WATCH! IT IS AWESOME.