Without getting too specific, the last series ended with one character travelling back in time four years to prevent a disaster, and the hero coming back to stop him to prevent the whole of space-time unravelling. So the net effect is that the disaster occured, leaving both characters with memories of four years of the future. Since their memories take in four years nobody else remembers, in this episode the hero keeps making mistakes such as thinking that his daughter is older than she is, his deputy is more experienced, etc.
What we end up with is the original sequence of events starting to fall apart unpredictably, with the effects made worse by the characters foreknowledge. They react wrongly to people because of this foreknowledge, and other people perceive this as undue familiarity or just plain oddness, making matters worse. There also turn out to be side-effects of the disaster that neither apparently remembers, including the guy who originally stopped it from happening - my guess here is that the new iteration was a little different, but there are other possibilities.
So what we end up with is a rather nice illustration of how difficult it would be to change the past, complete with uncertainty effects and random silliness. And at the end of it an apparent reset button... except that it isn't, quite. I think that this, and the other effects of the disaster, are going to give this season a much stronger story arc than the first.
Overall, a big thumbs up.
Now I really must watch season 1 again...