On the face of it, Swansong has the most ludicrous premise of any Fringe show – which needless to say, makes it very ludicrous indeed. The world has been consumed by a Biblical flood; four utterly incompatible people have miraculously survived, and somehow now find themselves on a swan-shaped pedalo. Why the flood happened, why this diverse group were together at the time, and how on Earth they ended up on the pedalo is never explained. But there’s a reason for all this weirdness, because Swansong isn’t just a post-apocalyptic musical comedy… it’s also a parody creation myth.

The clues are there in the opening song, with its monkish harmonies and references to ‘the legends of yesteryear’ – and even more overtly, in the incongruous stained-glass window hanging above the stage. Overall though, this faux-religious angle is subtly played and, perhaps, a little too easy to miss. What you can’t overlook is the comic banter between the mismatched survivors, who must overcome their differences to ensure survival and reboot the human race.

The characters are broadly drawn, but none the worse for that: we have Adam (dull but sensible), Claire (fitness fanatic), Stephen (Bullingdon Club) and Bobby (right-on New Age hippie). Much of the humour derives from the inevitable clashes between them, and everyone will find someone here who sets their teeth on edge. In the early scenes, the dialogue comes alarmingly close to being annoying for real; for the record, my personal fingernails-down-the-blackboard moment came when Bobby quoted the lyrics to American Pie followed by the poem Kubla Khan, and got both of them slightly wrong.

Over time, though, the mood changes, and the tone grows quite touching by the end. The negative character traits all have their flip-sides: the bon-vivant Stephen can be surprisingly comradely, while the frenetic Claire has the spirit to drive the group on. Ultimately, then, this is a kind of parable – of the power of circumstance to overcome our differences, and how mediating our beliefs can help us find unlikely friends.

The a cappella songs are less of a feature than they’ve been in some of DugOut Theatre’s previous work, but they serve as engaging punctuation for the spoken scenes. One of them, a close-harmony tribute to the majesty of a swan, hits the high-water mark of humour with its ending. During a later number, evoking a religious ceremony based on the four people’s journey, I appreciated the respectful interaction with the front row: you could choose whether or not you wanted to participate, but even if you turned it down you were acknowledged and, in a sense, involved.

All in all, I’ve got mixed feelings about Swansong. For me, the subtlety of the religious motifs sits uneasily with the broad-brush humour built around character stereotypes – and some mid-play philosophising, about whether it’s important to remember the bad things as well as the good, gets a little lost too. But in the end, it’s an uplifting story with a highly relevant moral, and there’s plenty of laughs to enjoy along the way.