Signal was one of the best kdrama I have seen. Apparently the serial killer part is based off a real crime that occurred in the late 80's. If you don't want spoilers I recommend you don't go much further because I'm going to write lots of spoilers. But you should watch the trailer.
When I wrote about the first two episodes earlier this season I was wrong and rights about the series. Yes this is a series about changing the past, but it wasn't about saving his classmate. It was something deeper. Park Hae Young was a very bitter young man who didn't trust the police and yet he became a police officer. This seems a contradiction until you find out about his past and then it all becomes clear. This is a young man whose family was destroyed because of corruption in the justice system and all he wants is the truth. As a child his older brother was framed for a crime he didn't commit and their family was torn apart. So the older brother in order to bring his family together again sought to clear his name and ended up being murdered and it was made to look a suicide. And this is why Park Hae Young did not go to a teacher or the police right away when he had witnessed his classmate being abducted. He was afraid. He didn't have any trust and when he did try to talk to someone he was not even given a chance. This is what made him the bitter man he has become. He is someone desperate for the truth. So when he finds the radio calling his name in the disposal truck this is a chance for him to try to save a lost loved one.
Lee Jae Han was a police officer with a deep sense of justice. From early in his career he received a strange transmission on his radio from an officer in the future giving him clues to help him try to solve crimes. Though things don't always go well for him. In fact a woman he cared for became the victim of a serial killer. However, the words of the young man on the other end of the radio about how cases became cold cases because someone gave up inspires him to seek the truth despite any opposition he comes across. He finds that things aren't so clear cut in the police department and that there is quite a bit of corruption. Those with money and power are able to get around justice and even cause those who are innocent to suffer. Yet he is desperate to get justice for the victims even if it costs him his life and happiness. And when he is about to expose some of the truth he is murdered and his desperation is what sets the cycle of transmissions between the past and the future. The past isn't set. Things can be changed.
And yes the two are able to change things with the help of Park Hae Young's senior officer, and Lee Jae Han's junior officer, Cha Soo Hyun. She is a capable officer who is in love with Lee Jae Han and for 15 years has been searching for him. Even though she is told that Lee Jae Han has run away after committing a crime she doesn't believe it. She is able to gain Park Hae Young's trust and eventually finds out about the transmissions. These three people are all connected through work as well as through the case that caused Park Hae Young's brothers case.
Even though they can't save the older brother they are able to change history to the point that the Park family is reunited and Lee Jae Han is saved, though he still disappears and is framed for a crime. However, things are very different from before. What we have in the end is hope. Even though we still see the person behind the corruption is still operating and trying to stop Lee Jae Han, we can see that the past has changed. In fact it had changed from the beginning because the first transmission said that Park Hae Young had told Lee Jae Han to go to the mental hospital but now it was Cha Soo Hyun. So it wasn't for nothing. Things could change. In the end we see Cha Soo Hyun and Park Hae Young on their way to meet Lee Jae Han. We are told they are going to see the person they have longed for. The one she loves and the one who is Park Hae Youngs best friend. That they don't know what awaits them and that perhaps it's tragedy but they need to go. It's also implied that perhaps there is someone else now who has the radio. Since Jae Han didn't die, he still had the radio and thus Hae Young never had it in this time line. So perhaps the date February 5th is another turning point for our heroes. One where it could be tragedy or another where they have another step towards gaining justice and punishing those who use people for their own gains.
The way it ended I could see it having a second season. It was really good. The music is really lovely and stirs the emotions. The film is interesting too. The past has a sepia color to it. The present has a more natural tint. Though it's not a very bright world. This is not a happy world. It's a world of injustice and sorrow. In the end though we are given hope even in the darkness.