Songfacts®:

• Here, Alt-J vocalist Joe Newman sings about a summertime pool party that takes a dark turn:
  
  Lifeless back slaps the surface of the pool
  Pool killer, killer, pool, pool killer
  
  The track seems to echo themes from the band's 2012 song "Bloodflood" that also mixes a violent story with watery images. ("A wave, an awesome wave. That rushes skin and widens in blooded veins.")
• The song was originally written in the band's early days at Leeds University. They completed it in 2016 in London. Alt-J said:
  
  "The brass was recorded at Abbey Road; the keyboards were done on a Casiotone that cost £1.05 on eBay; and no-one is quite sure where the key change came from."
• The music video opens with a voiceover by Iggy Pop before following the life of a wood mouse. Alt-J said: "We love working with great directors, giving them the creative freedom to interpret our tracks as they wish. It leads to great collaborations, like this new video for 'In Cold Blood' from Casper Balslev. As you are about to find out, a day in the life of a wood mouse can be unexpectedly dangerous."
• The song title comes from the 1966 book of the same name by Truman Capote. However, Alt-J admitted to NPR's All Things Considered, this was "mostly because it sounded good rather than due to the song being about mass murder."
• Alt-J reference American pop artist Jeff Koons' animal sculptures.
  
  Whiskey soda please, your G&T's empty
  Jeff's inflatables have sunk to the bottom
  
  Koons' artworks often resemble large, shiny balloons, but they're actually made of steel, hence why they've sunk to the bottom of the pool.