Think of Queen and what springs to mind tends to be big – their Live Aid triumph. Freddie Mercury's outrageous stage performances. Bohemian Rhapsody. Yet lurking on 1975's A Night at the Opera – the same album that gave us Bohemian Rhapsody – is a little-known gem of a track that shows us the band's quieter, more contemplative side.

'39 is a haunting folk song that tells the story of a man who goes on a journey and returns having aged "but a year", to find that that the lover he left behind has died in the many years which have passed. The track was written by Brian May, who had given up his PhD in astronomy for rock music, and though it's not made explicitly clear in the song, '39's protagonist has travelled through space and has experienced the time dilation effect of Einstein's theory of special relativity.

Of course it's also about a lot more than that, as Brian May explained in an interview for the book The Guitar Greats: "I felt a little like that about my home at the time, having been away and seen this vastly different world of rock music which was totally different from the way I was brought up. People may not generally admit it but I think that when most people write songs there is more than one level to them – they'll be about one thing on the surface, but underneath they're probably trying, maybe even unconsciously, to say something about their own life, their own experience – and in nearly all my stuff, there is a personal feeling."

The themes May explores in '39 – lost love, regret and the passing of time – recur in some of his other Queen tracks, notably All Dead, All Dead, Sail Away Sweet Sister and Leaving Home Ain't Easy.

On the studio version of '39, May plays acoustic guitar and takes lead vocals – only the second time he had done so on a Queen track – with Mercury and Taylor providing swirling, otherworldly backing and harmonies. The combination brings a joy to the track which belies the sad story of time-crossed lovers.

Live, Queen found another facet to '39, playing it as a raucous, rollicking sea shanty – this clip from Houston in 1977 shows Freddie Mercury on lead vocals and maracas and Roger Taylor on tambourine. But it's the album version that offers a glimpse of a very different Queen to the one you thought you knew.