A: To review: Just 1 More Thing has you draw an extra card at the end of your turn if you have an Investigator in play. This ends your turn immediately, and then you have to discard down to the Hand Limit if there is one. The mention of both Hand Limits and your turn ending immediately may be confusing, but, unless you’re mixing decks, you’re really only going to have to deal with one of those two issues at any given time. Here are a couple of sample situations:
Example 1: You have played all the cards in your hand, either because it’s Play All, or because you had fewer cards in your hand than you were allowed to play (starting with a hand of 1, Draw 1, Play 4, for example)… but you have an Investigator, and Just 1 More Thing is in play.
So you must now draw an extra card. Your turn now ends immediately, meaning you don’t get to play that card, even though the Play Rule would otherwise allow it. That’s the main thing that “your turn ends immediately” means: no more Plays, no more Free Actions. The only Hand Limit in Mystery Fluxx is Hand Limit 3, so the only times you’re going to have to invoke that immediate turn end, you’ll only have 1 card in your hand, anyhow, and won’t have to discard, so the Hand Limit is moot.
Example 2: It’s Play 2, Hand Limit 3. You have played your 2 cards, and have 3 cards left in your hand, and normally wouldn’t have to discard any… but you have an Investigator, and Just 1 More Thing is in play.
So you must now draw an extra card, and your turn ends immediately. There’s no confusion about whether you get to play that card, since you’ve already played your 2 for your turn (though note you can’t take any other Free Actions once you draw that 1 More Thing, either). In this case, the important thing to note is that you draw that extra card BEFORE having to discard down for the Hand Limit. After having drawn it, you have 4 cards in your hand, and must discard 1.