VITAL COSMIC ENERGY IS BEING DRAINED FROM THE Time Lords’ planet. The Time Lords’ unknown adversary is on the other side of a black hole, in a universe of antimatter. The President of the Council is about to seek help from the Doctor on Earth when he learns that the exiled renegade has been targeted by an antimatter scout organism. The creature traps the Doctor in his TARDIS at UNIT headquarters, forcing him to send an SOS.
No other Time Lords can be spared to assist the Doctor, so the President authorises the extraction of the Doctor’s two earlier incarnations from their own parts of his personal time stream, enabling the Doctor to help himself. Unfortunately, the First Doctor’s transportation unit becomes trapped in a time eddy. There is only enough temporal energy to project his image on the TARDIS scanner, but he is able to advise the Second and Third Doctors that the scout organism is a time bridge – and they must cross it.