In the middle of a summer heatwave, Explosives Officers ("EXPO") Lana Washington and Joel Nutkins are called out to a Metropolitan Police counterterrorism operation at a London housing estate to investigate a potential bomb factory. There is no sign of the bomb maker but with evidence of an even deadlier terrorist threat, Lana and Joel must work against the clock to save as many lives as possible. The first bomb is revealed to be simple fireworks in the toilet of a flat. The second is found on Andy Phelan in the boot of his car. Lana and Joel manage to disarm this bomb manually. Joel assists in investigating a suspicious parked van, which detonates, hiding a third bomb; Joel and several bystanders are killed.

Four days after the bombing at the housing estate, Lana and Danny respond to a suspected bomb on a bus, which turns out to be a false alarm. Following Joel’s funeral, Lana and her colleagues drink away their sorrows. New reports of a bombing points the police to an abandoned factory, which they discover is used as a bomb factory. Lana suspects some wires to have been planted, while Youngblood advises her to tread carefully. Far right extremists take to the street and blame islamists for the bombing. This later proves contrary as a bomb is discovered at a mosque. The countdown is already at five minutes when Lana goes in to defuse it. She also discovers someone behind the door the bomb is attached to. With three minutes left on the timer, Youngblood rushes in and orders her to leave the mosque. She faces the dilemma of saving the man behind the door or leaving immediately.

With seconds left on the bomb’s timer, Lana desperately attempts to unlock the door. Danny rushes in when it hits 20 seconds and convinces her to get out before it detonates, killing the captured man. Lana shares her experience with her friend, Karl. She also has an outburst at one of her colleagues, John Hudson, blaming him for not getting backup. The police investigate a cryptic message left by a right-wing splinter group, The Crusaders, which claims responsibility for the mosque bombing. Lana and Sonya discover that the bomb material is highly classified, and the former asks Youngblood to look into it. Another bomb is discovered outside an LGBT bar, The Five Oaks. With smoke grenades as a distraction against a sniper, Lana and Danny manage to disarm the bomb. Armed police storm the block of flats where the sniper was located, but only manage to kill the bomber, Nic Roberts. An officer is killed by a second suspect, having just discovered a police vest. Lana suspects that the second suspect is an insider, and even floats the possibility of a third suspect, a bomb maker. She breaks into Hudson’s locker, and discovers a map book marked with the locations of the bombings.

The police discover the burned out remains of the Prius that the bomber stole. Lana presents the evidence from Hudson’s locker to Youngblood, who promises to look into it. Her brother Billy misses a meeting with the police, and she starts looking for him. She and her parents discover his flat is empty. Lana takes her suspicions against John to the next level when she directly accuses him at New Scotland Yard, leading to DSU Marianne Hamilton asking her to take leave. Shortly afterwards, she is brought back in again after commander Bregman informs her that her brother has ties with The Crusaders and Roberts. Billy discovers a bomb in his glovebox when Lana contacts him. She directs him away from an underground station and into Cranstead Fields. Reinforcements arrive, and Lana attempts to defuse the bomb. With the timer activated, Billy starts to panic when it counts down from 30 seconds. Despite Lana’s warnings, he opens the door, leading to the bomb detonating and killing him.

Lana spends time grieving Billy’s death, and tries to drink away her sorrows and eventually decides to start therapy on the advice of Hamilton. She also tackles a conflicting statement with Youngblood regarding who she was with just before being called by the police. Another bomb is discovered at the University of South London, which is hosting a debate for a by-election. Lana goes in with Hudson to disarm the bomb. In the meantime, armed police spot the bomber, Frank Welsh, with two phones to detonate the bomb. They’re authorised to shoot him. As Hudson leaves, he detonates the bomb via his phone, and escapes. Evidence from his home leads them to another bomb at a ‘progressive alliance’ office, with a bomb located in a nearby car. Lana discovers gas triggers at the office, planted there and in the surrounding houses.

The bomb is defused. Hamilton takes Lana in as an advisor for the investigation, just as Hudson makes contact via text message, sending her the numbers 661142. They discover these to match the coordinates of an MOD black site that stores explosives of the same kind used in the prior attacks. The police secure the Deptford Town Hall after deducing that the attacks targeted the campaign in the by-election. Hudson’s unconscious body is discovered in St Jude’s chapel. As the Labour candidate is declared the by-election winner, Lana’s friend Karl Maguire takes the stage and reveals that he has a bomb. He reveals his intentions as revenge for the coverup of an ambush in Afghanistan back in 2009. The Labour candidate, Aleysha Campbell-Khan, was the chair of the Defence Select Committee at the time, who he blames for the coverup. Lana manages to talk to Maguire as SCO19 find positions. One of them manages to get a shot on Maguire, killing him. Hudson reawakens, his motive revealed to be to become a senior like Lana. Her team is called into preliminary hearings after the incident.