When Tom and Twigs invite cave-dwelling, light-sensitive dark sprites Rootle and Shade to come with them above ground, they use a Shadowbot to shield them from the light. Things go well until the machine runs out of power. As the shadow starts to shrink, the dark sprites panic. Tom uses BWM to power it back up, but then Twigs puts a red crystal into the Shadowbot's power cell, causing it to run off and spread permanent darkness wherever it goes. It's up to Tom to stop it, and quickly!