Crossover one-shot between Mazinkaiser and Shin Getter Robo. It was included in Immortal Super Robot Encyclopedia (不滅のスーパーロボット大全).
The manga begins with the Mazinger Z and Great Mazinger fighting against an alien machine after the Diana A and Venus A have been taken down.
(Source: Mazinger Wikia)

In one of their epic battles, the Mazinger Team suffers a crushing defeat at the hands of Dr. Hell and his minions after Mazinger Z is stolen and its pilot, the legendary Kouji Kabuto, is rendered missing in action. Just when Dr. Hell's second-in-command Baron Ashura uses Mazinger Z against our heroes, Kouji makes a surprise return in Mazinkaiser - an all-new Mazinger with heavier armor and deadlier attacks.
 
(Source: Anime News Network)

Shin Getter Robo is the third Getter Robo series chronologically. 
Years after the original story and in a brief time of peace, Professor Saotome has completed his masterpiece, the Shin Getter Robo, while humanity makes large advancements in space exploration and Getter Rays research. Unfortunately, this progress brings them to the attention of a mysterious and hostile race from across time and space. 
 
Note: Includes an extra chapter that was published on the last volume.

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