Songfacts®:

• Mutya Keisha Siobhan are an English girl group based in London. The group consists of Mutya Buena, Keisha Buchanan and Siobhán Donaghy, who are the founding members of Sugababes. Formed in 1998, the original lineup released the singles "Overload," "New Year," "Run For Cover" and "Soul Sound" before each of the trio individually departed the group at some stage. In July 2012, it was officially confirmed that the group had reformed under the name Mutya Keisha Siobhan and in December of that year they performed an intimate gig at Ponystep's New Year's Ever party, marking the first time the trio had performed together in over a decade.
• This song, which Mutya Keisha Siobhan penned with Dev Hynes, was released as their debut single under their new moniker. Mutya Buena told BBC Newsbeat that the trio took their time to select the right track to release. "We wanted to take our time and make sure it was right. The most important thing for us was to make sure we did it properly. [Flatline is] a lot more grown up, obviously still soulful."
  
  British songwriter and producer Dev Hynes' resume includes previous collaborations with Florence and the Machine, The Chemical Brothers and Solange Knowles. He has also recorded his own material under the monikers of Lightspeed Champion and Blood Orange.
• The song features backing vocals from Fred MacPherson, the frontman of the indie rock band Spector. Buchanan revealed in a behind-the-scenes look at the making of the song that the trio tried to record the song's "oohs" themselves, but it wasn't "manly enough." The trio therefore decided to get some male vocalists - including Fred Macpherson - to record them instead.
• The video was directed by the fashion photographer KT Auleta, who shoots for Vogue. It was her first music promo. Talking about the clip on Leona Lewis' blog Lee-Loy, Keisha Buchanan explained it was filmed in LA on Venice Beach. "We felt it was only right to do it there because 'Flatline' was written there," she said. "The concept is basically about friendship, girls hanging out and having fun. We wanted it to be positive - even though the song's about heartache, it's also about overcoming so we wanted a bright and positive feel."
• The song only reached #50 on the UK singles chart. Keisha Buchanan told Heat magazine the trio had other priorities for the track. "For us, our focus was always about making good music and making sure we weren't crap when we came back," she said. "For a single that didn't have radio support we were surprised that it charted. I'd be lying if I said it isn't about chart positions, but it's about so many other things as well. It got great reviews and our fans loved it."
• Mutya Buena, Keisha Buchanan and Siobhán Donaghy recorded "Flatline" for the second album by the original Sugababes lineup. The record remained unreleased until December 24, 2022, when the three girls dropped it as a Christmas gift to their fans. They released The Lost Tapes under their original Sugababes moniker when the trio regained the right to use the group's name in 2019.