Astrid Peth was a native of the planet Sto who worked as a waitress aboard the Titanic.
Astrid was a feisty young woman, who longed to discover the wonders of the universe and set foot on other worlds. She was thrilled when the Doctor granted her wish by allowing her to visit Earth. She was unhappy with her job as a waitress on the Titanic and was hurt by Rickston's harsh words for her.
She formed a close bond with the passenger Bannakaffalatta and was distraught when he sacrificed his life to destroy the Host threatening the survivors. She also began to experience romantic feelings for the Doctor. They kissed twice and she ultimately gave her life to save Earth and him from Max Capricorn's plot.
For most of her life, Astrid dreamed of seeing other planets. She met the Tenth Doctor when she broke a glass and he offered to help her clean it up. She refused. When he told her he was a stowaway and she offered to get him another glass of Champagne, he decided to take her on a teleport trip down to Earth.
During the course of the story Astrid asked the Doctor if she could be his companion and he agreed. Before they parted she kissed him, claiming it was a tradition.
To save the Doctor and defeat Max Capricorn, she lifted Capricorn and his life support system with a forklift and threw him into the engines. The forklift's brakes were cut by a halo thrown by a Heavenly Host. She plunged to her death along with Capricorn, but was dematerialised by an automatic function of her teleport bracelet. Using the advanced transmat system, the Doctor tried to recover Astrid, but only succeeded in creating a partially cohesive form, neither truly alive nor dead, trapped in a non-corporeal form for all eternity. He allowed her atoms to scatter into space, fulfilling her dream of travelling the galaxy.