Story: Three detectives in their late 20's transform to 70 year old men overnight after getting in contact with a weird liquid. Only the fourth member of the team, Park Jung-woo, doesn't transform into an old man for an unknown reason. While trying to get back their youth and finding an option to become their original age again, the Grandpa unit together with Park Jung-woo and Jung Eun-ji solve crime cases with Lee Joon-hyuk's intellect, Jeon Kang-seok's strength and Han Won-bin's charm.
Cast: For obvious reasons - there are so many people in the cast that it is really difficult to name and rate all of them separately.
Lee Joon-hyuk was portrayed by Lee Soon-jae and Choi Jin-hyuk in the different ages. Personally I think they did show a difference in portraying the character. While Choi Jin-hyuk made Joon-hyuk seem more cold than Lee Soon-jae, they both made him a little bit sarcastic and therefore someone others would hate but I used to like especially for this. The difference in their portrayal doesn't really bother me much since it is something also about the actors and what the audience wants to connect with them.
Byun Hee-bong and Park Min-woo both acted as Kang Won-bin who made this drama hilarious in his own way. Being a flower boy in his first age Park Min-woo portrays the shallow and popular guy very well as a little arrogant and obsessed with his own looks and skin care. Byun Hee-bong did well showing the personal crisis he experienced when Won-bin noticed he got old - wrinkly skin and not the usually perfect hair style was a horrible thing for him. Both actors did well while showing the arrogant and shallow aspects - as well as Won-bin being after pretty much every beautiful female.
In the role of Jeon Kang-seok Jang Kwang and Park Doo-sik both did well. As the character is strong but not really bright he had his own moments to shine and make the audience laugh. The most interesting thing about him was the fact that the character loved A Pink which led to a really cute behavior and a lot of aegyo the actors showed.
Kim Hee-chul of Super Junior is in this drama as Park Jung-woo and I personally think his acting was really good in the emotional parts. He did a very good job in this series and I think I would watch other comedy dramas with him, too.
Lee Cho-hee as Jung Eun-ji was really good, too. Her love for food and her ability to tell smells apart easily seems just a little weird and not really useful for a crime part at first even though it is entertaining but she turns out to be very helpful. As for the acting it was very solid, it somehow worked together even though I wasn't blown away by her acting.
Kim Eung-soo as the leader of the investigation unit Kim Young-chul was very interesting in his grumpy acting while also at some point caring for team members and their safety. The acting was really nice to watch.
And I need to mention Lee Ki-woo as Park Tae-min. I can't help it but I think after The Virus I developed a small weakness for Lee Ki-woo as the intelligent but slightly... crazy doctor or scientist. I think he is doing well in those rather serious roles and I was so happy when I saw him in this drama that I was almost jumping up and down because I got so excited.
Other actors in this drama series include Park Eun-ji as Han Yoo-ra, Kim Byung-se as Choi Jae-wook and Park Hyo-joo as Jun Hae-jin.
Soundtrack: I personally did like the instrumentals in the drama but not so much the songs by K-Pop/K-Indie artists Ladies' Code, Rumble Fish and Sun Woo. It is not a big minus though since I loved the rest of the music.
Recommended to fans of comedy and crime in one!