The man who forged his reputation at Celtic where he scored 52 goals in three seasons was returning after two years at Nantes to do the unthinkable and join the enemy.
To refresh your memory, Celtic believed they had a deal that would see him don the green and white again, but Rangers manager Graeme Souness had other ideas, admitting "mischief" was on his mind when he swooped.
No one saw the deal coming except for Celtic fan Jim Kerr and his band Simple Minds, who were at a party with Johnston at the time.
"Maurice had done it on the quiet and nobody knew, but I reckon we as in Simple Minds were the first to know," he told Talksport in 2015.
And even they questioned it when Johnston broke the news to them. There was plenty of beer about so maybe he'd had one too many?
"He could have been drunk but he told us: 'I've signed for Rangers but don't tell anybody.'
"We just thought what's he talking about it. But there it was in the Sunday papers, so we must have been the first to know, so I'll never forget that," he joked.
And as fans on one side of the divide cried into their collective Tennents, Johnston got to work at Rangers, eventually scoring 31 goals in 76 games and winning two league titles to go with the 1986 triumph at Celtic.
He seemed to enjoy this goal against his boyhood club, too.