Can someone explain the end of NCIS season 7 please?

I LOVE NCIS and have seen the first 5 series 3 times over and the 6th series twice. I have just watched the 7th seried as the DVD was just released in the UK and i didnt really understand what happened at the end of the series. I lnow about shannon and kelly and pedro but who was the drug woman and what went on the colnel bell and pretty much the last two episodes. Also i havnt seen the 8th series yet because i want it on DVD so can you please not reveal any spoilers, thanks