Act I: A Bewildering Fate

Reach Mondstadt: Proceed to the bridge outside Mondstadt. A cutscene will play where Mona discusses astrology and foretells meeting the successor of an astronomer, who turns out to be Klee. Mona gives Klee a box, which appears empty, disappointing her.

Catch up with Klee and retrieve the box: Follow Klee eastward. A cutscene will trigger as you approach, indicating the box isn't immediately visible. You'll first need to deal with attacking Dendro Slimes.

Defeat the attacking Dendro slimes: Dendro Slimes are nature-based enemies that hide underground. Using Pyro attacks on them will trigger the 'Burning' status, dealing Pyro damage over time. After defeating them, another scene will play, revealing Klee has found the box while you were fighting.

Accompany Klee to return the box to Mona: Return to Mona by the bridge and give her the box. It's revealed to be empty, but the Diary Mona was holding was the actual content, leading to her disappointment. Mona then mentions needing a place to stay as she spent all her Mora getting to Mondstadt.

Speak to Katheryne and help Mona find a place to stay: Go to the Adventurers Guild and speak with Katheryne. She will inform you about a house with a magical seal that appears to be abandoned.

Head to the magical laboratory and check the seal: Follow the path southwest of the Adventurers Guild to find the house Katheryne mentioned. Interact with the door to trigger a cutscene, which concludes Act 1.