The main trio, alongside several Muggles Ruli invited over using a forum, investigates a cave believed to be tied to a recent incident where buildings disappear from the city and come back from nowhere. However, when they're exploring the cave, their party members mysteriously vanish one by one, until Hiro is the only one left.
Tropes:
- Creepy Cave: The place that Ruli suspects to be tied with the building-vanishing incidents is a small, dark cave where people thought to had been vanished suddenly come out. There's a well-lit area at the bottom where Gigasmon and the petrified statues of his victims reside.
- Degraded Boss: Gigasmon debuted in Frontier as one of the Evil Legendary Warriors and its Starter Villain.
- Dwindling Party: One by one, the exploration party's members were taken away by Gigasmon to be petrified until Hiro, Gammamon and Jellymon were the only ones left standing.
- Obliviously Evil: Gigasmon petrifies people as monuments for earth's demise. Turns out this was because he mistook a disaster movie trailer as the real thing.
- Taken for Granite: Gigasmon petrifies his victims and turns them into monuments for (what he believes is) earth's impending demise.
