In the opening scene of the show, David Budd is on a train in London with his two children. He notices a suspicious man discarding his phone and entering the toilet, and a nervous female train guard who is worried about a possible terrorist threat. David, a police officer, decides to investigate. The guard believes the man in the toilet is a threat, so David prepares to deal with him if necessary. However, the man emerges without any explosives and returns to his seat. David then checks the toilet and discovers a terrified woman named Nadia with a bomb strapped to her waist. He talks to her, learns that her husband is the suspicious man, and intervenes to calm her down and prevent her from detonating the device.