The sudden outburst of strange, alien hieroglyphics on all of Alpha's screens is followed by the sights of what looks like a huge space anemone. An Eagle sent to investigate is horribly compressed and propelled back towards Alpha as a white-hot missile. Then Alan Carter's colleague, Kelly, comes into contact with the anemone-like being. Back on Alpha, it is clear that Kelly has been taken over and possessed by the mysterious alien force. By the time that Kelly is able to communicate that the huge space 'brain' is not aggressive, but just trying to prevent a collision, it may be too late. Koenig has ordered an Eagle loaded with an enormous nuclear charge to be aimed at the creature.
Things are quiet on the moonbase, and off-duty Alphans are doing puzzles to pass the time. Suddenly all the monitors on the base are filled with streams of strange glyphs, cutting off communication. Scanners indicate that the source of the strange symbols is somewhere along orbital reference 397.
Eagle 1 is dispatched to investigate. The glyphs stop as suddenly as they appeared, and communication is restored. Eagle 1 reports a phenomenon pulsing with light and color. The ship is covered with a strange foam-like substance and contact is lost.
A rescue Eagle is sent to search for Eagle 1 and is nearly hit by a meteor on a direct path towards Alpha. Inexplicably it passes through the defense screens and impacts near one of the launch pads.
Fearing that the meteor may have collided with Eagle 1, Kelly is told to do a space walk to look for wreckage. The meteor is retrieved and brought back to the base for analysis. Helena's research reveals the presence of an organic compound on the exterior of the meteor. Victor reports the composition of the meteor to be the same as an Eagle, along with traces of human tissue.
Realizing that the meteor is eagle 1, Alan is recalled to base. But he has lost contact with Kelly who was doing a space walk and won't return until he has him back on board. Alan finds Kelly floating unconscious, brings him back to the ship, and returns to Alpha.
Victor tells John that the organic substance coating the asteroid has reverted to it's original state, a "foam" that, in sufficient quantities, could crush anything. Kelly regains consciousness and begins transmitting data to the object in space. In an attempt to stop him, the computer is shut down.
Kelly attacks Koenig and Security personnal are forced to stun him. Koenig decides to send an Eagle loaded with nuclear charges to reduce the crushing force of the object in space that threatens their survival.
When the transmitted data is analyzed, it is surmised that the force is trying to change the course of the Moon to avoid a collision. In order to better understand the entity that has taken Kelly over, Koenig decides to link minds with him, via symbiosis.
Koenig learns that it is a brain, and it does want to help. Together with computer it comes up with a plan, but it will require the nuclear charges on the Eagle that's on it's way to the brain. Paul turns the Eagle around but the computer fails and the nuclear charges will not disarm, and they can no longer control it remotely.
Koenig flies out to dock with the Eagle, and disarm the charges, but is still unable to land it safely. He does manage, however to divert it from colliding with the base, but the nuclear charges are destroyed in the crash.
Unable to change their course, the Moon continues on towards the space brain. It begins to send out tendrils of the foam-like antibodies. The Alphans come up with a plan. They will attempt to equalize the pressure by increasing the internal pressure within the base, but it must be done gradually or it could be dangerous.
The foam starts to build up causing decompression in certain areas around the base. Medical Centre is compromised and during the evacuation, the unconcious, Kelly is overcome. The foam begins to flood the base, filling the underground hangars, the halls, and even Main Mission.
The Moon tears through the brain like a bullet. The brain dies and the foam is neutralized. They have survived, but at what cost? Helena mourns the loss of Kelly. She feels that she abandoned him when Medical Centre was evacuated. John tells her that she can't assume responsibility for Kelly's death, any more than they can assume responsibility for the death of the space brain. If only they had been able to communicate with it sooner, things might have been different.
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