This series has an unusually smart plot. The main character travels through time unintentionally, as it eventually turns out, to solve crimes that have been committed by a serial killer. What makes the story unusual (even original) is that unlike in most time-travel plots he travels to the future, not to the past. Surprisingly this works out pretty well. What is also unusual for a thriller is that the suffering of the families of the victims is much more central to the story than usual. This is in line with the main idea of the show, according to which police detectives are not simply supposed to catch criminals but rather to make sure that they cannot perpetrate further crimes. While I think that this aspect was exaggerated in Tunnel for the emotional effect, I think the concept as such is certainly commendable and still worked reasonably well in the plot.
The writers handled the obligatory plot twists very well. The viewer is led down several garden paths (most importantly concerning the identity of the main character's child and the identity of the serial killer in the future) and starts wondering about apparent consistency mistakes that the writers of the story made, but then everything falls nicely into place.
There were some things I didn't like. For one, it was too long and sometimes became slightly boring and slow, especially toward the end. The same story could have been told in about half as many episodes without omitting anything important and would have been more effective that way. Many unreasonable setbacks in the investigation could have been avoided if the main characters had communicated with each other, especially if they had told each other about their hunches and what they are planning to do next. This was at times very annoying. Another annoying thing was that the main character was constantly and needlessly saying and doing things that were giving away that he came from the past. While I suspect that this was supposed to function as comic relief, I didn't think it was funny but simply unrealistic.
While there were similar annoyances, I think this was a particularly good and intelligent thriller series. The plot twists were unexpected but believable, the story was unpredictable but not overly complicated. The acting was rather good not only by the main but also by the side characters. Finally, what I liked best was that the teenagers from the past who grew up to be the bad guys in the future were cast so that they resemble their grown-up self remarkably. This was done almost as well as in Looper and shows the effort that went into making this series.
