Of all of the episodes of the original series, next-generation, Deep Space 9, Voyager and enterprise, this episode is Star Trek. This episode is in fact what Star Trek is about.
Maybe this is an homage to the original series episode with "Lazarus", the man from two universes- but where that episode deals with time in respect to alternate universes, this episode deals with time itself, The constancy of time, and what would happen if that constancy were interfered with.
It also deals with time in a relationship that Picard had with a woman played here by Michelle Phillips, Who he meets here 15 years after he walked away.
Picard's former love interest is married to a doctor Mannheim, who has discovered a crack in time, a crack which has to be sealed.
Where normal people would be confused by alleged time-hiccups, Data is not confused. So Data is the one who has to work with past and future iterations of himself to close this crack.
This episode is always fascinating to watch, it's one of Brent Spiner's greatest Data performances. There is a quality of believability here, suspension of disbelief is not a problem, it's a fantastic concept, we believe in it because of its uniqueness.
Episodes like this were rare in the Trek franchise, but when they were revealed, they were very refreshing. In fact next generation had a double roll with the very next episode "Conspiracy".
Of course the last few episodes of season one lacked the appearance of Denise Crosby as Tasha Yar- Who I admit I did not like in season one but I actually liked a lot more when she appeared in "yesterday's enterprise" and then later as Sela, the daughter of that alternate Tasha. And those future episodes of course also dealt with time and/or alternate time-lines.
When we watch episodes like this, we start to wonder if time is as immutable as we think it is.
