"You did a lot of things," Napier said, "but you did them in a sort of 
unconscious, dreamlike state. You were sleepwalking. When we figured that 
out—after doing the biopsy on your fellow-troglodyte—we realised 
that in some sense you were no longer acting of your own free will, and we 
engineered a hunter-killer that would seek out and destroy the nanosites in 
your brain. We introduced it, in a dormant mode, into this female Drummer's 
system, then reintroduced her to your colony. When you had sex with 
her—well, you can work out the rest for yourself."