Required level: 40
Reward: Star Flute.
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To begin it find Lorelei in Cerulean, on top of Misty Gym and click on her:
She will ask you to help her to extinguish the fire in her friend's Dom house which is located in Vermilion:
When arriving talk with Dom and get in his house to extinguish the flames in the rooms within a certain time. (Bring Water or Flying type pokémon, with many area attacks, bring revives so you don't have to wait for your pokémon cooldowns).
After doing so, go back and talk with Lorelei to receive your reward.