 The boys and Candace return to a muddy area where the T. rex has left a massive footprint , which they recognize will eventually be on display in the museum in the present . Phineas leaves a message in the footprint for Isabella and the Fireside Girls to save them . The girls — in the present — immediately spot the footprint and the message and follow their handbook to create a new time machine . They use the machine to travel back in time and rescue the three . Upon arrival in the present , though , they realize a T. rex returned with them , so Candace flees and tries to expose it to her parents . A stray ray from Doofenshmirtz 's machine freezes the creature , so her parents merely believe it is an exhibit . 
