'System Shock', written by Justin Richards, features the Fourth Doctor, Sarah, and Harry.
Back Cover Blurb
'We're dicing with death on the information superhighway to hell.'
A rebellion on another planet. A kidnapping in central London. The head of MI5 assassinated. A hostage siege suddenly and violently ended by the SAS. A computer CD slipped into the Doctor's pocket by a dead man...
It's 1998, and the global information superhighway is about to come on line. OffNet controls everything digital from cars to sliding doors, from interactive television to military command and control systems.
The Doctor and Sarah must join forces wiht an old friend in a race against time: to prevent the breakdown of techonological society and foil an inconventional alien takeover bid.