A Doctor and Sarah Jane Smith fanfic.
It has been over a decade since The Doctor left Sarah Jane back on Earth, but she is still finding things a little hard without him. She has taken it upon herself to fight aliens that come to earth, hoping that she might one day run into him. One day Sarah Jane witnesses a large rock fall to earth near where she lives and decides to investigate it herself. When she takes a sample of the rock a purple gas is released from it, it goes straight into Sarah Jane's body. At that moment the Tardis materialises and the tenth Doctor steps out of it.
The Doctor doesn't figure out that Sarah Jane has been taken over until it is almost too late for both of them. After a series of unusual events Sarah Jane finds herself at home, not remembering meeting the Doctor again at all, but this time she isn't the only one left behind.