A silver instrument balloon has landed in a bird sanctuary. When the warden, Ollis, investigates, he vanishes in a crackling flash of light.
Dr. Tyler takes the instrument to UNIT. He recently detected an intense beam of energy directed at the Earth and is seeking an explanation. The Doctor has Tyler develop his latest plate while he investigates the sanctuary. Tyler develops the plate and sees the image of Ollis on it. He then opens the instrument box and vanishes in a flash of light. A glowing blob of energy emerges.
The Doctor detects radiation at the bird sanctuary. The Brigadier has Benton search for Tyler.
When the Doctor returns to UNIT, the energy blob emerges and attacks. He and Jo flee, but Bessie vanishes in a flash of light.
The Doctor confers with the Brigadier in his lab. He sees the image of Ollis in the plate, detects radiation from the box to the drains and reads reports from other scientists. He realizes that the energy beam searched Earth and then sent the blob specifically after him. The others were attacked by mistake.
As they ponder their next move, gel creatures suddenly appear and advance on UNIT HQ. The UNIT soldiers are outclassed in the ensuing fight and the Brigadier orders an evacuation. The Doctor urges Jo to leave with Benton, but when the energy blob returns, they have to take shelter in the TARDIS. He alerts the Time Lords to the danger.
The Time Lords, however, are losing power to a nearby black hole from which the energy beam has emerged. Faced with such a menace, the Chancellor decides to send the Second Doctor forward in time to help the Third.
The Second Doctor materializes in the TARDIS. He explains the dilemma facing the Time Lords. But they quickly begin to argue about their next step.
The Time Lords decide to send the First Doctor to keep them in order. But he is caught in a time eddy and only able to appear on the monitor. He explains that the energy blob is a bridge and urges them to cross it.
The Second Doctor switches the force field off and the Third Doctor races out into the lab. Jo follows him and the two vanish in a flash of light…