I've watched two amazing episodes from season 4 now, The Visitor and Hard Time. So many times Star Trek tackles serious topics but it comes off as cheesy and cliche, but Hard Time just nails it with excellent writing and an incredibly restrained and nuanced performance by Colm Meaney, plus some excellent support from Siddig, Chao, and Brooks. It's classic both as sci fi and pure drama. It tackles several themes expertly and takes the show to the next level. I'm not going to recap the episode (there are a couple of other threads here that will capably review it when they get to season 4), I just thought it was outstanding.