Commercials don't get much sadder than this two-minute clip for St. John Ambulance, which portrays a father who bravely defeats cancer only to die of choking while his daughter looks on. The soundtrack for the depressing ad is Sia's 'I'm in Here.'

The somber tune may be a surprise to new fans of the Australian singer-songwriter, who are used to hearing her vocals in up-tempo dance-pop tracks by the likes of David Guetta and Flo Rida.

'I'm in Here' appeared on Sia's 2010 album 'We Are Born' and seems to be an appropriate choice for the powerful commercial. As Sia coos, "I need you to hold all of the sadness I cannot, living inside of me," viewers see the heartbreaking story of a young girl who watches her dad survive treatment for cancer before choking on a burger at a backyard picnic.

The point of the commercial is to remind people that the lack of simple first aid skills kills as many people annually as cancer, and to encourage viewers to sign up to receive a first aid guide. The ad certainly makes an impression — we won't be able to hear this Sia song again without picturing the sad commercial.

Hear Sia's 'I'm in Here' in the St. John Ambulance First Aid Commercial