Partners in Time is the second Mario & Luigi rpg and it is by far the strangest and IMO by far the most underrated.
The villains are pretty fun in this one. The Shroobs are alien mushrooms, and they act kind of like a kid friendly version of the Mars Attacks aliens. It's great.
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Also Mario & Luigi fight by recklessly using their baby selves as weapons. I told you this one is weird.
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I'm lucky I grew up with the Aus region version of this game, however, because I hear the US version inflated the bosses HP to the point where the battles last way to long.
It's surprising how dark partners in time gets. If it weren't for the cartoony art style it would be way too disturbing for kids to play. Such a shame it never got remastered because it's the first M&L game I played and it's my favorite one.