When he didn't show up in London, Mandy deduced his fate, and cried silently in London Market. The Doctor and Amy watched from the TARDIS scanner. Mandy was subsequently followed by Amy, and, when she confronted her, she had to warn Amy not to enter a tent marked "KEEP OUT". During this time, she noted Amy's Scottish accent and informed her that Scotland had wanted their own ship rather than be a part of Starship UK.
Amy ignored her and came back in shock, then was knocked out by the Winders. Mandy waited for her outside the voting booth with the Doctor. When the Doctor pressed the Protest button, Mandy was locked outside, whilst Amy and the Doctor dropped down a chute to the star whale "below".
Queen Elizabeth X approached her in a mask, and Mandy joined her in collecting the Doctor and Amy. She was then taken to the Tower of London, where she was reunited with Timmy. A part of the star whale approached the two children, and was gentle with them. (TV: The Beast Below)
Mandy was a very clever, kind and caring young girl. When her best friend Timmy didn’t meet her in the London market deck she couldn’t stop herself from crying as she had easily guessed that he had “been sent below”.
While she was initially fearful and obedient like the rest of the residents of Starship UK, once she quickly got roped into Doctor and Amy’s adventure, Mandy proved to be very brave, calm, helpful and insightful. Her sole motivation was to find her missing friend and she was elated to find him alive in the Tower of London. (TV: The Beast Below)