When I did mine, I did a recap of prior storylines, and a bio on the characters, but I think it can be very effective to start without that, and go right into the story. All that backstory can be a little dry and boring.
I'll point out that almost none of the big story hours had much in the way of backstory. Piratecat's Story Hour had the most established PC's, but still just had a brief post explaining what the PC's were up to at that point. All the other major ones I recall (Sagiro, Wulf, (contact), etc) started right in on the story - though they had the advantage of starting at the beginning of their respective campaigns.