UFO Robo Grendizer 3.15

Destroy the Marine Base

or Suicidal Explosion of the Undersea Base

or Opération plongée (Diving operation)

Zuril gets pushed by Vega Daio to finish off Grendizer. Zuril launches a swarm of bomb carrying balloons. They strike the Space Science Center. Professor Umon orders Kouji, Maria, and Hikaru to head out in Double Spazer, Drill Spazer, and Marine Spazer and deal with the balloons. They do just that and find the source of the balloons but Grendizer shows up when things get to be a little too intense for them.

Grendizer and Marine Spazer combine and dive underwater in pursuit of the vessel that was launching the balloons. They are attacked by two saucer beast but they manage to make it back to surface and then back to the Space Science Center.

Professor Umon reveals that he is working on a new vehicle, an Ultra Submarine. Grensizer is only able to dive to 400 meters but with the Ultra Submarine Grendizer will able be able to go as deep as 3,000 meters. The professor orders everyone to stay put but when the Vegans launch rockets at the Space Science Center, Kouji disobeys orders and launches in Double Spazer. Maria follows him in Drill Spazer. 

Kouji gets captured by the Vegans. They treaten to kill him if Grendizer does not meet them as their base which is at 400 meters beneath the seas. Kouji breaks free and warns the Space Science Center that it is a trap. The base at 400 meters is a decoy. The real base is 1,000 meters beneath the sea.

Grendizer, Marine Spazer, and Drill Spazer come for Kouji. They bring with them the Ultra Submarine which has been completed just in time for the battle. Grendizer and the other use the Ultra Submarine to destroy the two saucer beasts and rescue Kouji. They also destroy the base at 1,000 meters. The episode ends with Zuril promising to get revenge on Grendizer for this loss.

Thoughts

This is a key episode according to Wikirak, probably because the underwater base was destroyed.

Is Gandal still alive? He was also in the underwater base but there was no sign of him after the base started to flood.

Danbei and Goro are in this episode. They are in a scene that takes place at the Shirakaba ranch before the bomb carrying balloons showed up. They aren't seen after that.

This episode moved very fast and there was a lot packed into it. It is significant in the bigger picture of the show but it is mostly memorable for the milestones.