Argyle new boy Akos Buzsaky, on loan from European champions Porto, forced a save from Preston goalkeeper Andy Lonergan and crashed a shot against the bar.
And Scott Taylor's shot grazed the post before Preston took the lead against the run of play through Sedgwick's outrageous chip from the edge of the area.
Whether he intended the goal attempt, or whether it was a cross that drifted over Argyle goalkeeper Romain Larrieu, only Sedgwick will know.
Then, former Dundee United defender David Worrell sold Larrieu short with a terrible back-pass, allowing Agyemang to nip in between the pair and easily convert.
It was rough justice on Plymouth, who started the game in great form, with ex-Motherwell midfielder Keith Lasley, given a start after 11 consecutive games as an unused substitute, also bringing a save out of Lonergran.
Despite Argyle taking the game to the visitors in similar fashion during the second-half, Preston's well-disciplined defence held firm and successfully saw the time out.
Williamson, whose side briefly led the Coca-Cola Championship earlier in the season, now faces the task of keeping Argyle up, while Davies can plan with some confidence for a tilt at the play-offs for the Premiership.