„Tulejoonel“ on põnevik ja perekonnadraama, milles rulluvad lahti teravad kaasaegsed teemad ja uudsed tegelased. Sarja keskmes on Marko Matvere kehastatav 57-aastane Jakob Sommer, kelle töö- ja eraelu on karidele jooksnud. Kõrvaltvaatajaile sõbraliku ja rahulikuna näiv vanaisa tunneb ühel hetkel, et maailm tema ümber on jäänud võõraks ning ta otsustab seda plahvatusohtliku missiooniga „parandama“ hakata. Pealinna oma hirmuvalitsuse all hoidvat pommimeest asub jälitama noor kaitsepolitsei tippuurija, keda kehastab Kaspar Velberg. Lisaks teevad sarjas kaasa Sten Karpov, Mait Malmsten, Tõnis Niinemets, Liis Lass, Saara Pius, Märt Pius, Karin Tammaru, Märt Avandi jt. Eesti tippnäitlejad.

Mark and his team begin to investigate the bombing, while the man has to investigate internal control over his own working methods. Tensions in Mark's team increase when an armed and dangerous group of young people linked to a bomb blast is arrested. "Bomber" Jakob tries to avenge his former employer, but at the same time has to deal with his wife's work worries and suddenly finds a new target to detonate the next bomb. A new powerful explosion is raging in Tallinn.

Kapo's research team is under public pressure, investigating a new explosion that this time claimed human life. Mark gets a new clue about the explosions, leading his unit into a fierce exchange of fire with a local mafia group. Tensions in the research team are growing. At the same time, the "bomber" Jakob desperately needs a new explosive and comes across a dangerous opponent when it is procured. Suddenly, however, the enraged man now finds something very powerful that can take the crushing explosions to a whole new level.

The port manager comes out of the coma and tells investigator Mark why he ran in front of the car. It turns out that the man is a victim of brutal blackmail. Mark, together with the Criminal Police, organizes a raid to catch the blackmailer, during which a prostitution network is discovered that enslaves women who have come to work here from Ukraine and Belarus. At the same time, Jakob is confronted with a real-life group whose member he blew up and whose lives are in serious danger. There is a new and powerful explosion.

In the new explosion, random people were killed and public pressure puts more and more pressure on the research team - the case must be resolved quickly. At the same time, a young man is killed and Jakob stole the bomb. However, Jakob is raising money, the "bomber" doesn't care about his own well-being, but wants to leave something for his family. The attempt to blow up the ATM fails and DNA is found on the unexploded fertilizer bomb. The research team is on the verge of a breakthrough. Jakob compiles a list of enemies and parks a van full of explosives directly in front of the Riigikogu building.

A major operation takes place to capture the "bomb man". According to the DNA found in the fertilizer bomb, the bomber is Armin, Jacob's friend and former combatant from whom he stole the fertilizer. The operation goes totally bad and only then does Mark realize that the real "bomber" is still at large and planning something terrible. The race begins with time to hit Jakob, and another bomb will soon explode.