In this universe people are able to see each others souls. Everyone has a specific colour for their souls, and if you have a strong enough emotional bond with someone then their soul colour with start showing in yours. Dan Howell has been trying in vain to get the small streak of blue out of his soul, but nothing works. Painting his nails (which is one of the two places you can see the colour of someone's soul), never talking to anyone with a blue soul, even going as far as isolating himself from everyone, but nothing works. It mocks him, that streak of blue.
Phil Lester has been trying to get the streak of black out of his soul since 2012. He hasn't tried measures as extreme as Dan, but he has been trying. After hearing about a job opportunity to take pictures of models for Gilmore Magazines, he can't say no. Of course, he starts to rethink his decisions once he hears what model he has to take pictures of.
Through several hardships and an excessive amount of black nail polish, things start turning out for the better, though. Dan and Phil realise that they can't ignore the fact that they have intertwined souls.