Bui Bui was the monster of the week in Ep.40 of UFO Robot Grendizer. It's vehicle form was the plain standard flat saucers, color grey, with purple marking on top and bottom, while it's robot form resemble a stereotypical hunch humanoid type monster: a big purple and lavender torso with massive spikes running down its back and suction portholes all over the chest; long arms ending in claws, and short legs that ended in the saucer shells for feet, its extremities were dark colored with lavender rings; a hue pink skull cover in spike; dark abdomen and pink pelvis,

Bui possess eye beams, super strength, extendable arms and use its saucer feet for drilling; from the suctions holes on the torso it could suck rocks, magma, and other matter into them, using the two former to fire projectiles from the mouth while storing anything of importance in its body; Bui was capable of underwater combat. The Saucer shell provided protection from any of Grendizer's projectiles.

Side Note 1: Bui is also another creature converted into a Saucer Beast, based on the subtitles, it comes from the Floating Star, were creatures like it thrive on Uranium (I assume it refers to underwater Planet, since it is well known fact that the Japanese word for Star is also use for Planet and has been the subject of many jokes, and since it is a underwater based monster, makes it obvious.

When Underwater eruptions force the evacuation of an Uranium Mining facility on a nearby island, both the Space Center and Vega Invaders dispatch their forces to take the Uranium; the former to avoid a nuclear disaster, while the latter detects a even massive Uranium deposit underwater, which is what causing the geological event.

Side Note 2: Umon warns the risk of an Uranium explosion, which could lead to deadly radiation contamination in the Kanto Region, killing all life in the Kanto Region: keep that in mind for later.

Zuril and Gandal dispatch the Saucer Beast Bui Bui to collect the Uranium, since it is design for underwater missions.

Grendizer and the Double Spazer are dispatch to the island: once there, Duke uses Double Harken to dig the storage tank from the foundation, dealing with Earthquakes, Eruptions and Tsunamis hitting the island; Grendizer lifts the Tank and Combines with the D-Spazer before avoiding the a super eruption. At that moment, Bui Bui is detected entering the atmosphere and Team Grendizer is warned as the beast attacks. The beast catches up to the Team and tries to take the Uranium Tank, though is kept at bay by Spin Saucers, Melt Shower, Double Missiles and Spin Drill, with the Main Spazer providing cover to the heroes: this gives enough time to deliver the tank to another Island Base, were it is put in a huge vault, which Bui can't penetrate. the Beast continues to its intended mission, with Team Grendizer following it.

Bui arrives to the Eruption site and submerges and begins to dig; Vega detects Grendizer approaching, and Gandal decided to fight it, much to Zuril's disagreement, thinking the Uranium comes first, the former ignoring his protest. Duke, despite been almost without power, ejects from D-Spazer and and its grabbed by the Beast, been dragged underwater, where they fight: been almost depleted of energy and not design for underwater combat, Bui Bui over powers Grendizer, though it is distracted by incoming Magma from the nearby eruptions. Bui sucks Magma into the suction holes to fire Lava projectiles from its mouth and then pulls in Grendizer and gabs it, though the Mecha lift the Beast bashing it into the underwater mountains, though at the cost of losing energy faster. Bui beats Grendizer and Gandal is to give the order for the final blow, when the Skull Base begins to shutdown, having depleted it's main power source; Zuril takes charge and uses the Reserve Energy Capsule to re-energize the Base, and orders Bui Bui to return to its mission to collect Uranium, as Grendizer floats back to the surface.

Bui reaches the main magma vein and begins to extract the Uranium from it to the point of drying it and takes off, Duke ordering Koji to go after the Beast, much Hikaru's protest of helping Duke. D-Spazer attacks the Beast, cutting its spikes with Double Cutter in the fight: Zuril orders Bui to ignore them and escape, while Gandal order it to take the weaken Grendizer as hostage. Bui grabs Grendizer, but the Mecha, using its last reserves of Energy, throws Bui into the Water, Duke then fires Space Thunder straight to the monsters mouth, the electrical discharge causing a chain reaction, with Koji dealing the final blow with Double Cutter, causing a massive Explosion. Dealt another defeat, Gandal vows to eliminate them in the next attempt, Zuril finishing it off by saying they will eliminate Grendizer in the Ocean, having discover its other weakness.

Side Note 3: Yes, these chuckle fucks caused a massive nuclear explosion in the end – Ooooooops!

Later, Umon presents the plans for the next Spazer that will help Grendizer with underwater combat – the Marine Spazer, which Hikaru is chosen to be its pilot.

Side Note 4: Guess Umon decided ignore the fact these idiots did the one thing he wanted to avoid to happen that could kill all life in 100 kilometer. Oh well, the sensation of METAL IN YOUR TEETH doesn't seem so bad at the end of the day, huh Umon?.