Jan Jansen Summoned Home

This is a companion quest, don't delay too long or Jan Jansen will leave the party Fifteen days after Jan's joining, a messenger comes up to tell him to get home right now. High-tail it back to the Slums and to his house. You'll meet his lost love Lissa, who has returned with their daughter, the little girl is now deathly ill. Talk with his (one of the many) uncle to learn that you need to talk to the Hidden. For most of this quest, Jan stays at his house. To find the Hidden you must talk to Lady Jysstev in the Government District (3300 exp), it seems that the Hidden is willing to meet with you (despite you not being an "Enlightened One") in the sewer beneath the Copper Coronet (8900 exp).

Enter Copper Coronet, go into the back rooms, and down into the sewers, the Hidden will be on the southern end of the little sewerage river. Talk to him, but keep things no-nonsense and simple (he isn't fond of talking to you), he agrees to help the girl if you take care of his enemies for him. Trace the proprietor of the Sea's Bounty Tavern in the Docks district and pass the secret word on to learn that the two you seek are on the second floor of the Five Flagons Inn in the Bridge District. The two Gith will realize you are there to harm and response immediately. Return to the Hidden once they are dealt with (17,500 exp) and you may now ask questions about him, who will reveal himself as a Mind Flayer (Illithid).

Return to Jan and the girl will be healed. The plot continues downstairs (15,500 exp), where Jan asks your opinion on matters, agree to support him and he rejoins the team.

Sources & references

 * Baldur's Gate II Walkthrough by Dan Simpson