For MI5 officer Liz Carlyle, the nagging complications of her private life are quickly forgotten at Monday’s counter-terrorism meeting. An invisible may have entered mainland Britain. An ‘invisible’ is a terrorist who is a native of the target country, who can cross its borders unchecked and move about unnoticed – the ultimate nightmare. By the time they're identified, it may be too late. The intelligence operation that follows will test Liz to the limit. Who or what is the target? Where and who is the invisible? With each passing hour the danger increases. But as Liz desperately sifts through the incoming intelligence, she finally realises a truth that could prove deadly: her ability to get inside her enemy’s head is the only hope of averting disaster... But the clock is ticking - and anyone could be at risk.