... And then said their "final" goodbye.
Once Dougal was dead, and Rupert was on Jamie's trail, and the plan to poison the Bonnie Prince was yet another failure, it was time for the hard part: Jamie asked Claire to make good on her promise to avoid the coming bloodshed and return to Frank. She begged to stay and presumably die by his side, but since Jamie fucking tracks Claire's periods and knew she was pregnant again, he insisted she leave him forever. Rough.
Their final moments together at Craigh na Dun — frantic, passionate love-making session included — were classic "Outlander," even if pretty much everyone knowing they'd eventually reunite took away some of the sting from the bitter goodbye.
Still, this was a final goodbye of sorts, since when Jamie and Claire reunite next season, they'll have had 20 years of life to mourn, grow, and presumably move on on some level. Jamie will have a daughter he might never get to know, Claire will have the guilt of leaving, and both of them will have lost the innocence of youth.
It will be a very different "Outlander" with a very different Jamie and Claire next spring, and "Dragonfly in Amber" was both a worthy goodbye to the show that was, and an exciting reminder that the future — grey hair and all — could be just as bright.