The Doctor and Jo grant make a visit to a prison to see how the Keller Machine works.  This is a device that is supposed to be able to remove the criminal thoughts from individuals.  A prisoner collapses from the treatment.  At a World Peace Conference some documents disappear and a delegate from China dies mysteriously.  UNIT and the Brigadier are supposed to be provinding security for the conference.  The Doctor, having gone to the conference, helps the Brigadier stop an attempt to kill the American delegate by a Chinese assistant who is under the influence of the Master.

It is revealed that the Keller Machine is actually a housing for a mind parasite.  The Master uses this to cause trouble at the prison and then gets the convicts there to hijack the Thunderbolt missile being transported by Captiain Yates.  He plans to use it to start a new World War.  The Doctor takes the Keller Machine to the airfield where the missile has been taken.  He uses it to keep the Master occupied while he set the self destruct on the missle.  The Master escapes but the mind parasite appears to have been destroyed in the blast.