Phineas, Ferb & Sonic: Adventure is the sequel to Phineas and Ferb meet Sonic the Hedgehog where Phineas, Ferb and a lot of other characters from the show head back to Mobius. It's sequel is, Phineas, Ferb & Sonic: Vs. Mario, but it's chronological sequel is Phineas, Ferb & Sonic: Halloween