The pilot of Fringe will be familiar territory for fans of The X-Files as it establishes a small FBI team tasked with investigating strange phenomena. The episode "Peter", an hour-long flashback which explains that one of the protagonists was kidnapped as a child from an alternate universe by his father's doppelganger, is anything but familiar.
Fringe improved immeasurably once it began to explore the parallel universes, and by season three was alternating between the two, complete with separate title sequences. Fringe later went on to introduce alternate timelines and the evolved humans called Observers. If you had caught part of a season one episode, it would have been relatively easy to pick up on the episode's storyline and maybe even some of the overarching mysteries like Olivia's Cortexiphan abilities.
Dropping in on the complex and mythology-heavy seasons which followed is a more daunting task, but for longtime fans, Fringe's evolution is a reward in itself.