The 17-episode fifth season of Lost aired in winter and spring of 2009. It was the show's second to last season, and, as such, it finally begins to unravel some of the mysteries that have been set up throughout the first four seasons. This season also sees the survivors split not only in location but in time, since moving the island at the end of season four to protect it from the "freighter folk" resulted in time rips so that some characters are back in the 1950s, some are in the 1970s, while others remain in the present day. The show does finally start to explore the island's history, explain what the Dharma Initiative really is, reveals the 'incident' that led to requiring someone to press a button inside a hatch every 108 minutes, explains the time travel, explains why the castaways crashed on the island in the first place, and reveals more about Jacob. Some of the reveals are ham-handed and/or eye-roll-inducing, but some of them really pay off well for the die-hard fans of the show. This season, while again a bit shorter (17 overall episodes) does contain the milestone 100th episode of the series and does again end on a cliffhanger setting up the final season.
The Blu-Ray set is a five-disc set. As with the prior season releases, you get a good amount of bonus material, including commentary tracks on a few episodes, about seven making-of and behind-the-scenes featurettes, including one on the 100th episode, a series recap of seasons 1-4, and a gag reel. So, there's a lot there if you like watching the bonus features. Overall, the season is very good. It continues to be well acted and mostly (with some exceptions) well written. The writing does get a bit too clever for itself sometimes, but it does seem like the writers had almost everything planned out and were not just making stuff up on the fly. A big complaint by many fans really from the end of season two onward is that the show was not resolving the mysteries and introducing four or five new questions for every question it answered. I would not say that was the case this season. Major plot elements were explained this season, and while not everything gets wrapped up, you can definitely tell the series is winding down. You definitely need to watch the prior seasons for this one to make any sense as this show is truly a serial story that builds on itself, so if you just jump in randomly, you may enjoy it, but you will not have any idea what is really going on.