Using only two voices and two guitars, Alice Costelloe and Kacey Underwood create music that’s unfettered by the constraints of a traditional band. Their sound is gauzy but spartan; his vocal, dusky, hers crystalline, twinned in a spectral haze. From the first song they ever wrote together, the distractedly sweet ‘Homework’, to the celestial drift of ‘Pi’ Big Deal’s music is unflinchingly honest, powerful in its intimacy and quivering with the intensity of a first crush.

They come from two different worlds – Costelloe from an artistic London household, while Underwood was born into a strict religious family in the desert between Joshua Tree and the Yucca Valley. When Costelloe was 14 she played guitar in her first band; when Underwood was the same age, he found escape in music. Records were his key to freedom from the rules that surrounded him. He immersed himself in Metallica, Hendrix, Led Zeppelin and Depeche Mode and taught himself to play on a guitar he found under his brother’s bed.

At 19 after a stint as a guitar tech for surf legend Dick Dale, Underwood swapped California for Cambridge, leaving his old life behind and throwing himself into a Social and Political Science degree. But it was not to be; he left uni, eventually ending up in London fronting the band Little Death.

By contrast, Costelloe’s fascination with music was kick-started at six-years-old when her grandmother presented her with a guitar. “I still write on it now even though it’s tiny”. She credits her stepdad’s record collection – stocked with The Beach Boys, Blondie, Bowie, Elvis Costello and The Smiths – as being similarly formative. “When I was six my stepdad convinced me AC/DC were amazing and taught me to headbang,” explains Costelloe. “I think I started guitar because I wanted to impress him.”

And somehow these two disparate backgrounds converged – but not through a chance encounter on London’s gig circuit. While playing in Little Death Underwood taught guitar at a girls’ school to pay the bills and it was here that he met Costelloe’s mum who suggested he meet her daughter to help her brush up on her guitar skills.

As it happens, the first song Underwood taught Costelloe was Sonic Youth’s ‘Teenage Riot’, but after listening to Big Star’s ‘13’, with Underwood encouraging Costelloe to sing, lessons swiftly fell to the wayside and their songwriting partnership blossomed.

It was during this time, the shy swapping of ideas, melodies and lines, that Underwood and Costelloe’s respective bands began to disintegrate, leaving the duo in a surprisingly easy and natural creative space. They hung out, ate ice cream, had BBQs and watched the entire season of ‘Freaks and Geeks’ in a week. Underwood was embraced by and became part of the Costelloe clan.

Out of the bedroom and onto the stage the duo hid behind their hair while their songs inspired the rapt attention of their audience. Mute signed up early on the strength of the pair’s live shows and it’s easy to see why. Where so many bands rely on and hide behind onstage strut and pomp, amps-up-to-eleven, swathes of reverb and other expected performance pieces, Big Deal’s live shows are hard hitting in their restraint. Their stripped back, bedroom-composed pop songs and the duo’s clear chemistry are so mesmeric that a hush instantly descends on any audience, with those assembled eager to take in every nuance in their entwined voices, every lilt and lyrical twist. With one acoustic and one electric guitar, their songs carry an instant connection and while their performance is inclusive, it also leaves you with the sense that you’ve been privy to a secret exchange.

Eighteen months and many shows later, with a short break for Alice’s A Levels (the NME tour providing an unlikely revision aid), Big Deal have Lights Out. Co-produced by Underwood and Dean Reid, the duo recorded 15 tracks in just seven days, and the result is a cohesive, deeply personal and simultaneously universal collection. From the nostalgic sweep of ‘Distant Neighborhood’, to the deeply reflective 'Seraphine’, to the raw confessional ‘Talk’, their innate ability to tap into deeply felt emotions have lead many to assume Underwood and Costelloe are a couple.

“Most of the bands we like are pretty upfront and that sort of just influenced me to say what’s in my head,” says Costelloe.

“It’s totally understandable,” agrees Underwood. “If I was looking at a guy and girl singing onstage, singing the types of songs we sing, my imagination would go to them. Creatively we’re totally not looking to make music with other people.”

‘Locked Up’ expanded upon in the studio from its live simplicity to a wall-of-sound swoon, contains the poignant lyric: “I’m a mess / I’m a wreck / But you wouldn’t know ’cause I’m at my best when with you.” “I feel like it’s a song that sums up our friendship, and musically it’s an example of how well we work together,” says Costelloe. “I brought the song to Kacey without a clear direction for it and he took it somewhere that to me is perfect. I think our different backgrounds help us compliment each other. We come at problems from such different places that we end up always being able to solve them.”

And therein lies the key to their unlikely pairing.