When news of a terrifying army of rats pouring through the tunnels reaches the ears of the Shepherd's Bush Colony, a brave young mouse steps forward to lead them in a desperate race to escape the darkness and reach the safety of Hampstead Heath. The journey is fraught with danger, not only from the rats, but also from the infamous Whitechapel, an evil platform cat with a particular fondness for mouse suppers. Along the way, these brave young warriors face their fears, forge strong bonds of friendship and ultimately discover themselves and what it is to make a difference in this big, wide world.