Plot: Every night, 10,000 workers descend on the Tube to maintain, repair and clean it. We follow this invisible overnight army as they work against the clock in the four-hour window when the power is off - from hard-working Bulgarian cleaner Vladimir, who is amazed that the British government pays for people not to work, to 23-year-old Harry leading his gang of 'fluffers' picking fibres and lint from the tracks