Henry needs to get the Chrono Crystal from the Beast of Pluto.

Walk right from Grandpa's lab and talk to Spflornk of Pluto about the Beast.

Start by investigating your surroundings. You can find the place where the Beast is summoned.

Go down from Grandpa's lab to the Offering Site. Keep walking right. There's a set of steps up. (I nearly missed this entirely. It's difficult to see.) Go up the steps.

Search the Altar and get the Music Score.

Show the Music Score to Spflornk of Pluto. This is accessed via dialogue, once you have the Music Score.

You'll have to find a Beast Singer called The Silent One.