For the Dr Who it was this one http://en.wikipedia.org/wiki/T...
The probe is intercepted by a giant spaceship heading for Earth. When the broadcast is shown, an alien face appears and identifies itself as being a Sycorax. The alien demands Earth's surrender and causes a third of the world's population to go into a hypnotic state. The Sycorax threaten to make these people commit suicide unless they are given half of the world's population as slaves. One of the scientists discovers that all of the hypnotised people share the same blood type (A-positive), the same as contained in a sample on Guinevere One.
They used blood control to control people.
On the Ansible you are probably right.