Gertie was a companion of the Twelfth Doctor. In 1986, the Doctor dropped her off and told her he'd be back at 26 September, but he didn't tell her what year.
One of Gertie's adventures took place on Mars, during its earlier stages when it had water and thriving plant life. She explained to her grandson Charlie Baker that it was peaceful and quiet, until something interrupted them.
Once she encountered a Gestalt Creature which attempted to superheat the planet's surface and destroy all life.
Another time, the Doctor took her to the past to see a pyramid, yet they were attacked and chased by a mummy.