Just after finally defeating the Sontarans on modern-day Earth, the Doctor's TARDIS takes the Tenth Doctor, Donna Noble and Martha Jones on an unexpected trip to the barren planet Messaline. Arriving right in the middle of a war between humans and Hath, the Doctor meets someone he thought he never would: his daughter. Can the Doctor accept this clone as his offspring, and can he stop the war before it all ends in massacre on both sides?