Doi Doi was the Monster of the week who appears mid way in Ep. 38 of UFO Robot Grendizer: it was a humanoid type Beast, having a shell disk for a body, with a massive horn on top, with red, yellow and purple arms and light/dark purple legs appearing in between the shell. The Right arm ended on a hand while the left one had a whip; Red vertebra column and purple pelvis; it's feet had small red spikes. It had a yellow mark over the shell with a small Blue green head with antenna coming out of the center. The Vehicle form was the conventional flat disk, with the big horn protruding from its end, interestingly, the head did not hid in the shell, so it was visible in vehicle form. Besides its whip, which release an energy discharge as well spin it fast enough to create a protective shield, it would fire an energy attack from its antenna (it bee compose of Vegatron Energy) and the horn, as well ram its enemies with it.

As the energy crisis cause by the loss of the Vegatron Star becomes of paramount concern for the Invaders, King Vega demands his commanders (General Gandal and Minister Zuril) to acquire Uranium at any cost. On Earth, Umon, Koji and Daisuke can only wonder why the invaders were after the Uranium, Daisuke (correctly) concluding that something must've happened to their principle power source. Realizing this, Umon states a new war has been waged and the enemies attacks will more savage than before.

On the farm, Hikaru, trying to prove herself tames and rides the wildest stallion, much against her father's orders. Daisuke catches up to her, demanding answers for her behavior, which replies she is capable of helping more since been taught how to pilot the D-Spazer. Realizing she is training to fight the enemy, Daisuke dares Hikaru to follow him, their chase leading up to a rapid river: Daisuke jumps it with his horse making to the other side, but Hikaru, stops on time in fear. Daisuke then tells her the Battle ahead will be ruthless, not wanting her to get involved.

Side Note 1: Yeah, you have Koji and an entire Science Lab involved in this war as it is, and the family you work with are often the targets of these aliens – I think we have long since pass the "I don't want to get you involve in this fight", m'uh dude!

As Hikaru is reprimanded by her father about her wild ride with Daisuke (calling him lazy), Goro (been the only other one to know the truth) comes out in their defense, almost spilling the beams on Daisuke, though halted by Hikaru. Been corner by both his children, Dambei drops the argument. At the lab, Daisuke Umon and Koji look into space with special sun glasses at energy sparkles in the sky: Photo Squamton (NOT JOKING that's what they call it), which would serve as power source for both Grendizer and D-Spazer, been it more powerful and less harmful than Uranium, and soon learn a large quantity of it is over the atmosphere, and they decided to collect some, but understanding Vega could detect their actions, Duke launches Grendizer to be a distraction while Koji flew in the Atmosphere to collect the Energy samples. The two launch with Grendizer been detected in orbit between Earth and the Moon, and Gandal orders a Minifo strike force and Beast Doi Doi to intervene. Zuril other hand, takes reason understand why would Duke do this.

As the fight ensues with Duke decimating the Minifos and confronting Doi, Koji flies in the atmosphere collecting Photo Squamton energy particles with his device, as Umon watches the procedure with Hikaru and Goro. As the fight continues, Zuril launches his flagship (which kinda resembles the Mothernburn) to investigate what other motives exist in this surprise attack, soon concluding it to be a diversion and his vessel, along with a strike force, divert their attention to Earth, soon discovering Koji's D-Spazer in the Atmosphere, ordering the fleet to capture it. In their fight they zap Koji with an electrical discharge, though he disposes of some of the Minifos with Double Cutter, a weaken Koji pilots back to base safely but stun to be of any assistance. The Mechas take their battle to the Planet, the Beast proves to be too formidable for Duke, and with Koji still out of commission, Hikaru pilots the D-Spazer without permission to assist Duke. Goro also wants to join too when he spill some stuff to his father about Hikaru and Duke: Dambei confuse after everything that has happened, he demands answers, but Goro rides off.

Side Note 2: So yes, seeing what has been happening between Hikaru as of late and Daisuke, Dambei puts two and two together and solves Daisuke true identity as Duke by his own, albeit off screen.

During the fight, Doi fires Vegatron Energy from its antenna, causing pain to Duke, when D-Spazer arrives, firing Cyclone Beams, the beast falls down the cliff, and Duke suggest to combine (thinking it is Koji). Their first attempt to connect fails, Duke states to time it correctly, and in the second try both he and Hikaru connect. As Doi climbs back up, it tries to capture Grendizer with its tentacle, but Duke grabs it and fires Screw Punch, which shreds the tentacle down and through the Beast's body, disposing of it quickly. Hikaru sheds tears having connected, with Duke realizing it was her piloting D-Spazer.

Side Note 3: WHICH BY THE WAY, Hikaru crying for her success in connecting is the title drop of this episode – "A Tearful Connection with Hikaru".

After the battle, Dambei demands answers from Daisuke, now knowing the whole truth, feeling dishonor cause no one trusted him, he TRIES to commit SEPPUKU but is stopped by Koji, telling him he should be proud of Hikaru, which makes him cry even more. Daisuke now realizes she has effectively join the team in the fight for Earth.