The Guardians look after time for all people. Humans always rush around claiming that they do not have enough time, but no one thinks of guarding it. The Guardians do this and in every region of the world there is one who protects time for us all.
In Greenwich, it is Old Father Tim. When the Wreccas, who populate the Underneath (below ground), send Snot to steal the Tick, their only intention is to wreak havoc on the Guardians who live Topside (above ground). They don't expect Snot to find out that she feels more at home Topside and that she really rather likes Tid (Old Father Tim's grandson) who she has to trick.
And little do they know that without the Tick, time will stop forever . . .