The Wandering Monster of Sorrow (かなしみのさすらい怪獣 Kanashimi no Sasurai kaiju) is the 10th episode of Ultraman Leo.
A kaiju arrives and Gen recognizes it as an old friend. Can he defeat it?
MAC spots a meteorite coming in on earth and detects a lifeform on it. They investigate it, and indeed find a monster, Ron. Gen is unable to fire at it, because he remembers it as a pet he had in L77. The kaiju escapes underground. Dan knows the secret of the monster, but still wishes for Gen to kill it.
Gen sees Momoko punish a child, Kaoru, for being mean because she was lonely, and begins to understand Dan's point of view. Earthquakes are experienced because of Ron moving in the earth's core. MAC is set up outside the volcano where Ron is expected to appear when Gen shows up, and is put on radar duty. The monster appears and downs several planes. Gen attacks, and when his vehicle is crushed, transforms.
Ron apparently calms, but attacks Leo with his tail. They fought, but Leo managed to shrink his former pet to its original size and bring it to space. Later, Gen gives Kaoru a puppy named Ron.
Ultraman Leo Volume 2 features episodes 9-12, & Complete DVD-Box Features All Episodes.