Next to the body of her murdered uncle, Ena finds a strange puzzle piece, but she can't remember how she got there. She becomes a profiler to find out the truth, but detective Hansaem remains convinced that she is the killer.

Ena receives a second puzzle piece shortly before discovering another murder victim. Detective Hansaem suspects it is not a coincidence and begins investigating while Ena tries to find the link between the two.

Ena begins working at the Hangang Police Station to try and solve a case she believes is linked to her uncle's murder ten years ago. A wary Hansaem starts collaborating with her and a prime suspect emerges.

Ena and Hansaem set off to investigate a mysterious body found in a reservoir. Just when they think the case is closed, Ena gets caught off guard by something unexpected.

A third puzzle piece arrives just as a locked-room murder occurs. As the duo draws closer to catching the killer, they agree to work more closely to uncover the real truth behind the puzzle murders.

Hansaem and Ena try to uncover the meaning behind the drawings on the puzzle pieces and the killer strikes again just as they identify the next victim.

Hansaem and Ena continue their investigation and past events come to light, shocking them both. Ena goes back to square one and Hansaem is torn when he hears about the suspect in another murder.

The investigation grows after another victim is discovered. Ena and Hansaem each set off after a different suspect.

In an unexpected twist, a new suspect is arrested shortly before another victim is discovered. Ena and Hansaem risk it all to try and catch the true killer.

Hansaem and Ena believe they know who the murderer is and rush to stop them.

As the final puzzle piece arrives, have Ena and Hansaem lost their last chance to catch the killer?