Circumstances Part 1

When a seemingly random Riolu wakes Clemont up in the middle of the night and led him out into Lumiose City in the pouring rain, the last thing he expected to see was Ash Ketchum and Pikachu sitting in an alleyway, completely soaked and with no memory of how they got there. What happened to them? And why does Clemont's heart skip a beat when Ash smiles at him?