The ship's surgeon you rescued has proven to be such a phenomenal storyteller that Charles Johnson would actually prefer his account to whatever Low could come up with. So now the continued existence of Low is his main problem - so he wants you to see to it that Low's testimony cannot contradict the story that he is planning to write.
This is a sea battle against three enemy ships, one level 50 flagship and two escorts (both between level 43 and 46). You only have to defeat the flagship.
Possibly a minor continuity error: the pirate Captain who Johnson sent you to find in the last mission (Bad Medicine), and whose name was Lowther, is now being called Low throughout this mission and in the follow-up. Johnson could just refer to Lowther under a (presumably) well-known pseudonym, but there is no indication of this in the mission text.
- Defeat Captain Low