Four Dimensions

The Doctor, Alice and Jones were getting used to having ARC around. A shapeshifter whose origins are shrouded in mystery, ARC still saved their lives time after time. So why has ARC suddenly commandeered the TARDIS, crossing back over its own timeline in one of the most dangerous stunts a time traveler can pull?! Has ARC been a sleeper agent all along, or are there deeper motives at work? It's up to the Doctor and his companions to find out – if ARC doesn't wipe them all out first!