Once upon a time, Danny was 'Dangerous Dan', an I'll-try-anything-once hedonist, but he's recently decided it's time to grow up.
He's been promoted in his pet insurance job, he's 'getting his big boy on' - and he's getting married to the girl of his dreams, Kate. He's also decided not to ask Milo to be best man at his wedding in favour of the extraordinary bore Paul.
His adventure with Milo through parallel worlds forces him to evaluate their friendship and think carefully about whether he is really ready to settle down and marry.
Milo is a full-time stoner and Danny's best friend since they were kids.
He has been brought up by his granny, Jenny, as his parents died when he was little.
Since Jenny died, Milo has lived on his own in her house, turning it into a stoner cave and refusing to engage in real life.
He is devastated when Danny chooses Paul to be best man instead of him. As he travels through different worlds and meets different versions of himself, Milo is forced to take a cold, hard look at some of the decisions he has made in life and decide whether it's time now for a change.
Kate is Danny's long-term girlfriend and now fiancée. She desperately wants Danny to start behaving like an adult and knows that he won't until he stops getting high with Milo.
There's no love lost between those two - Milo blames her for Danny's conversion to Mr Sensible and Kate blames Milo for constantly trying to derail things for Danny.
Callum is trying to kill Danny and Milo... but we're not quite sure why.
When he first appears out of thin air, a wristband Danny and Milo find on his arm throws them into a parallel world where everything seems drastically different. The only chance that either of them has of getting home is to find another wristband.
If that weren't complicated enough, it turns out that Callum is in this world too and still wants to kill them...