Peter Hamilton, London Underground's managing director, is horrified when his ex-wife is pushed under a train. Following the murder of a second commuter, he receives an anonymous phone call from an organization calling itself Vortex that is dedicated to preventing the privatization of the network: 'You were the intended victim... Next time you won't be so lucky.' Hamilton turns for help to Lyle Revel and Hermione Bradbury, a glamourous couple with a talent for solving murders. But as the death toll rises, the terrorists release a runaway train on the network...