I haven't been the biggest fan of Season 4, but I must say, the last four or five episodes have been way more entertaining than the start of the season. This episode seems to be the pinnacle of the season, occurring right at the middle and combining everything (minus the conspiracies) that the show does best.
The basic idea is that a man is killed in a car accident, losing his head (literally) at the same time. However, later that day, he gets up, headless, and walks out of the morgue. Mulder and Scully begin investigating and from here, we get our usual X-Files format: Scully believes it's body snatchers for profit and Mulder believes its full body regeneration. He compares this man, Leonard Betts, to a salamander or a worm who can regenerate body parts.
The episode itself may seem a bit boilerplate, but it had plenty of humor, more than its fair share of eerie moments and an ending that hints at sinister things to come. Also, did Mulder invent the whole "will the real Slim Shady please stand up line" two or three years before it even came out? When he asked the real cancer man to please stand up, I burst out laughing since this is happening three years before Eminem came onto the radio.
Definitely a great episode of X-Files and proof that stand-alone plots can be better than the alien/conspiracy stuff sometimes.