"Gretna - A Different League" will be broadcast on BBC Two Scotland, Tuesday 8 May, 2100BST
David Irons took over from Rowan Alexander for two months
Rowan Alexander says that physical and mental exhaustion forced him to miss out on Gretna's run-in to their historic promotion to the SPL.
The manager had been signed off sick for two months by the time they clinched their third straight title with a final-day, injury-time goal.
"I felt worn out," Alexander says in the BBC documentary "Gretna - A Different League".
"I had six years non-stop 24/7 and felt really run down. It was stress."
Alexander admitted that he had not delegated enough responsibility to his backroom team.
But club owner Brooks Mileson had persuaded him that he had to take a step back for the sake of his health.
"Brooks advised me to take time off and I've never doubted his advice and honesty," said Alexander.
I have total belief that I will be manager of the football club this time next year
Gretna manager Rowan Alexander
Tuesday evening's documentary explains how Alexander's departure for what at the time was an unspecified illness came days after it was announced that goalkeeper Alan Main would not play for the club again.
It had been revealed the week before that Main had signed a pre-contract agreement to join promotion rivals St Johnstone next season.
Gretna lost their next game 3-0 at home to relegation-threatened neighbours Queen of the South and Main appeared to get the blame.
There were reports of dressing-room splits as Alexander relinquished control to assistant Davie Irons.
And speculation has followed that Irons or director of football Mick Wadsworth - Sir Bobby Robson's assistant with England and Newcastle United - would be in charge next season.
But Alexander, who pointed out that he had spoken to Irons every day during his absence, again insisted that he will return as manager.
"I would not have signed a five-year contract if I didn't think that," he said.
"And Brooks would not have given me a five-year contract otherwise.
"I have total belief that I will be manager of the football club this time next year."
The documentary charts Gretna's season, which saw them come from third place in October to lead by 12 points by January before being a matter of seconds away from finishing second to St Johnstone on the final day.
Mileson also talks of his own health problems, an operation following a burst bowel and internal bleeding forcing him away from the club from July until the end of last year.
The owner insists that cost-cutting in January was required to "build a team for the future and not just for a season" as several older players were released or dropped out of the team.
Even the host DJ was axed in anticipation of higher costs next season as the club are forced to share Motherwell's Fir Park because Raydale Park is not acceptable to the SPL.
Director Graeme Muir explained that many of the players and staff who took them to the First Division would not be good enough for life in the top-flight.
But, by the end of a difficult season, he could proudly pronounce: "This is the dream - little Gretna are in the SPL."