A Spy on Board is the twenty-seventh episode of Mobile Suit Gundam. It premiered on Japanese television on October 6, 1979, and in North America on August 27, 2001.
Amuro presents to the White Base crew on the various Gundam and G-Fighter upgrades and some proposals for improvement. Kai excuses himself during the presentation to go to the bathroom. Out in the hallway, he complains about the other crewmembers acting like they want to live on the ship the rest of their lives. General Revil thanks Amuro for the presentation, telling him that they will get to work on his proposed improvements. Once repairs are complete, White Base is to head to Jaburo.
Katz, Letz and Kikka spy on Kai as he changes into civilian clothes and walks off the base. Along the way he is spotted by Bright and Amuro. Kai says military formalities don't suit him and he's leaving. Amuro is surprised that he is leaving after they've been through so much together, but Bright says to let him do as he wishes. Kai says he's been pushed past his limits by the Federation and Bright. On his way out, Amuro brings Kai his toolbox, saying he can exchange it for some money. On the bridge, Bright calls Kai a coward.
Meanwhile, Boone's squadron is supplied with a Z'Gok mobile suit. Char says that they must stop the Trojan Horse from leaving and that they must know where its headed. Boone says he has someone working on it. Meanwhile, Kai runs into Miharu, who invites him to stay at her home with her and her younger brother and sister, Jill and Milly. Miharu brings her to the abandoned house that they live where they are greeted by her brother and sister, who are initially afraid of Kai. During dinner Kai talks about White Base.
When Miharu and her siblings leave the room, Kai looks through her basket and finds a gun inside. He comments on how sad it is and lets it slip that White Base is leaving and has had problems with one if its engines. Miharu immediately sends this information to Boone and Char. Char requests that Miharu sneak aboard the Trojan Horse during their next attack. Char departs on the Sea Lance and returns to the Mad Angler Squadron. The Z'Gok mobile suit is released and heads towards the harbor. Boone orders a man named Connolly to head to shore during the attack to inform Miharu of her orders. The Zeon ships bomb the harbor and release their Gogg mobile suit. Sayla heads out on the G-Fighter and Amuro heads out in the Gundam soon after.
Kai wakes up and notices that White Base is already being attacked. Jill and Milly tell him that Miharu has gone 'shopping'. Miharu meets up with Connolly, who tells her that her orders are to head aboard the Trojan Horse and find out where its headed. He gives her money, a uniform and a fake commander's letter. Boone, notices the G-Fighter and Gundam and has his mobile suits fire on them. Sayla fires into the water and is successful in hitting the enemy.
Miharu returns to her home and is told by the children that Kai has left and wished her luck. Miharu tells them that she will be going away for a while and gives them the money. Hayato heads out in the Guncannon as the Z'Gok nears White Base. Kai watches over the battle and is glad he got out. Hayato fires towards the Z'Gok but the Guncannon is quickly damaged by it. The Z'Gok pilot believes the Guncannon to be the Gundam and doesn't think it lives up to the rumors. Kai finds it hard to believe that he is worrying about White Base and thinks of the times the crewmembers have yelled at or hit him. He admits he is a coward.
Underwater, Amuro battles the Gogg and is able to destroy it. Fraw calls Amuro back to White Base to help out Amuro. Kai grabs someone's motorcycle and quickly heads back towards White Base. Hayato continues to have difficulty battling the Z'Gok. Kai rushes back into White Base and jumps into Guntank. The Z'Gok grabs onto Guncannon, but it is hit by blasts from Guntank. The Z'Gok pilot tries to force the Guntank into the water where it has an advantage. Amuro soon arrives in Gundam and the two head into the water.
Miharu, wearing a Federation uniform, is able to head into White Base unnoticed during the confusion. Sayla is able to successfully sink the enemy submarine with the G-Fighter. The Z'Gok pilot realizes that the Gundam is the true enemy mobile suit that he has heard about. Gundam is able to lure the Z'Gok out of the water and Kai blasts it with the Guntank. This enables Amuro to destroy it while it is stunned. The remaining Zeon forces withdraw. Kai returns to White Base and is welcomed back. On the Mad Angler, Mulligan tells Char that their spy has successfully made it on board White Base. Char orders Boone to keep up with White Base. Miharu hides in a storage room on White Base.
- Mecha Introduced: MSM-07 Z'Gok