MacDonald spent most of the second half of last season with the Home Park club and, after he'd played against them for Burnley in this season's Capital One Cup, returned for a second loan, this time until 6th October.
During this loan he's played eight times, scoring once in a 1-1 home draw against Southend. He was in the team on Saturday against AFC Wimbledon, the last day of his current loan.
That's now been extended to the maximum 93 days for an emergency loan which will see him remain with Plymouth until the 15th of December.
Alex had previously had loans in his adopted Scotland with Falkirk and Inverness Caledonian Thistle, and in his two loans he's now made 26 appearances for Plymouth scoring a total of five goals.
Given that at Burnley he was unlikely to be playing any competitive football whatsoever, he will continue to benefit from being at Plymouth.
Manager Carl Fletcher said today: "He's come in, settled in well and added something to the team and that's exactly what you want from loan players.
"Alex has been playing a little bit wider than normal but he's one of those players who can do a job anywhere and he seems to be relishing that position."
Just last week, MacDonald said he would consider a permanent move to Plymouth Argyle.