Theory:Doctor Who television discontinuity and plot holes/New Earth


 * Why, after locking the Doctor in a door to be injected with diseases, does Cassandra open the same door a few seconds later to let him out?
 * She wanted the contaminated patients out to stop Casp and Jatt from attacking her. She could only do so by also letting the Doctor out.


 * All of the Sisters' cures are intravenous solutions, yet the Doctor is able to cure everyone just by adding them to the disinfectant in the elevator. It is unlikely that an IV solution could be administered topically like that and be effective.
 * The Doctor added lots of other chemicals to the disinfectant, and the disinfectant was from the far future so it would be much more powerful than the 21st century type.
 * We already have chemicals today that can safely penetrate bodily tissues and carry other chemicals with them. The first, and still most popular in medicine, is DMSO, which has been used since November 1963 for everything from topical application of non-topical local anaesthetics to transporting anti-inflammatories to bone cavities without injections into the bone, as well as, allegedly, turning cyanide into a contact poison and LSD into a contact drug. DMSO doesn't magically transport every chemical exactly where it needs to go, many chemicals wouldn't make it into the bloodstream and many of those that did would get through the blood-brain barrier, with disastrous consequences. But it seems reasonable that by the year 5 billion, even better transport chemicals will be found, and will be as readily available in hospitals as DMSO is today.


 * In Father's Day (TV story), when Rose Tyler touches the baby Rose, a paradox is formed. However, when the dying Cassandra O'Brien.Δ17 in Chip's body gets touched by the younger Cassandra, no paradox is said to have been formed.
 * In Father's Day (TV story), the paradox is formed because 1) Rose saves Pete (it's never stated explicitly, but Pete's death may be a fixed point in time); 2) she does it in front of alternate versions of her and the Doctor. When the two Roses make contact, they worsen the paradox. So it's "reasonable" (if there is anything "reasonable" in a time travel story) that the contact between the two Cassandras does not create a paradox, or creates a paradox not strong enough to tear the fabric of reality. Alternatively, the paradox does not happen because the young Cassandra is in her real body and the old one is not.
 * Cassandra going back in time in Chip's body and telling herself that she's beautiful is a closed loop, which is a stable paradox. In Father's Day (TV story), the issue was them changing time and creating an unstable paradox.