Time, Space, Etc

Gareth from 2008 and Ayesha from 2100 disappear and find themselves in a totally different universe. And then another. And so on. Mostly, they spend their time either running for their lives, saving innocent bystanders or just generally disappearing at the most unhelpful moments, usually when they are about to find out something useful.
Is there a reason for all this randomness? Probably, but it's a secret kept with Arianna, the mysterious narrator, and she's not in the mood for talking.