The episode starts with an OZ base reporting an unidentified shuttle approaching (though it seems to be too small to be a shuttle). The OZ officers at the base issue the ship a warning and then open fire on it. The ship immediately turns into a Gundam and fires a massive blast at the base. The blast is unusually powerful, destroying everything in and around the base.
We go to Lady Une who is standing with Tubarov in inspection of Mercurius and Vayeate. They receive word on what happened in the previous scene and we learn that the Gundam was actually a new or modified one - OZ didn't have any records of it. Lady Une orders the dispatch of Mercurius and Vayeate. Tubarov doesn't agree, he wants to send the mobile dolls.
Trowa covertly goes to the 5 scientists' cell and tells them about the new Gundam. They figure out that Quatre has made the modifications based on an old plan they were always too hesitant to implement. Trowa then goes to Heero's cell where Duo and Chang are also kept. Trowa takes Heero to pilot Mercurius and beats up on Duo a bit for show, dropping a gadget that projects messages from the scientists into his hands in the process.
Heero and Trowa launch into space but they're too late. Quatre arrives at another colony and destroys it. Heero and Trowa change course.
We turn to the diabolical Tubarov. Escorted by two guards, he shuts off the oxygen to all the prisoners on board the station (including the scientists and Chang/Duo). One of the guard's questions that he is acting against Lady Une's wishes but Tubarov simply states that the Lady has gone too soft. The prisoners have approximately 30 minutes to live.
In space, Heero and Trowa (along with a formidable army of OZ mobile suits) come upon Quatre. Quatre destroys all the mobile suits, leaving only Heero and Trowa alive. Trowa is overjoyed - the new Gundam is so powerful that all three of them can join forces and fight OZ. Trowa approaches Quatre to give his plan but Quatre fires at him. Quatre wants to destroy every weapon in space - regardless of who or what controls it. He has gone mad with grief about his father and sister's death.
The episode ends with Heero taking out a plasma saber and announcing his intention to kill Quatre.