The story picks up at the point where a cheerleader club is looking for new members.
Little do the new ones know, that the cheerleader club is almost closing due to prophecies and controversies.
Captain Jungwoo is trying his best to save the club.
On his mission, he meets Haeyi as well as Sunho, who are joining the club for ulterior motives.
During their time in the cheerleader club, strange things are happening, which are linked to a past accident.
And so the mystery unravels. Slowly.
Or... at least it should.
Typically, the focus should be on solving the mystery and revealing the truth little by little.
Yet, there is a long drought until you actually forget, what the whole show is about anymore.
The writer clearly had his focus on the love triangle of Haeyi, Jungwoo and Sunho to the point, where even the mother of Haeyi and her brother are rooting for either of them.

I knew what I signed up for. But I didn't know, how bad it could have been implemented.
While I enjoyed tiny bits of the love triangle, it was just a pain to watch overall.
Haeyi, a bubbly, energetic girl yet childish and immature.
Her entire character didn't grow// there was no character development at all.
Sunhos story plot is well-developed, his character was interesting. I was rooting for him for the first quarter of the show, until he showed his obsessive side. Since then, he has been a red flag walking on two legs.
Jungwoo was as fascinating as a piece of plain white paper. No character depth in his character, no real background information. He just existed in the drama and happen to be the main lead.

Main leads beside, the side cast was great and I really enjoyed watching them dork around and rooting for them, even more than for the main lead couple.

Overall, the show was a mess.
I thought the title reflects it content.
A happy show that lifts my mood.
The drama started well and I was intrigued, but it lost its interest fast.

Cheerleader content? There are rarely any scenes. It's mostly just practice, which showed the same move.
Mystery? Literally butchered down. And those tiny bits we got are underperforming.
Romance? Yeah. No. Chemistry of the main leads was missing.

The only saving points were the side characters, their friendship and their story arcs.

Cheer up should have been a happy, motivating show, yet it left many of us frustrated, wanting to finish fast or drop it.