Sammy's Super Sonic Adventure is the crossover novel between the Sammy Space-Time and Sonic series. Though it is not considered the third book in the Sammy Space-Time series, it is canonical in the Sammy Space-Time series and the events of Enter Sammy Space-Time lead directly into it. It is complete, and it can be read by clicking here.
After Sammy embarked on a new quest at the ending of Enter Sammy Space-Time, he was led to the Gateway to the Multiverse because he had no destination in mind. There, he met Shelby (who he now has a crush on) and she explained where he was and how he got there. He browsed around the strange and surreal realm for a while until he noticed a portal to Sonic's universe, which he entered.
Later, while Sammy was in Green Hill Zone, a group of Badniks began menacingly approaching him. Luckily, Sonic came on the nick of time to fight off the Badniks. Sammy offered to tag along with Sonic to Emerald City, and Sonic eventually agreed to let him come. At Emerald City, they requested Tails to help stop whatever Dr. Eggman was up to, and Tails accepted. However, Tails devised a plan to retrieve Knuckles, Shadow, Amy, and Mighty's help as well so that they'll have more of a chance against Robotnik.
They went from Angel Island to the GUN fortress, to Amy's house, and then to Tumbleweed Trails (a town within Frontier Canyon Zone). After recruiting Mighty, Dr. Eggman captured Sammy and took him to his base so that he could use Sammy's power to reshape history. While Eggman was busy finishing the final touches on the Egg TMRE-Channeling Champion, the team completed the series of trials leading to Eggman's "throne room". However, the entire team was captured before entering the room except Sonic and Tails. Once at Eggman's "throne room", Eggman revealed that the Tails that was accompanying Sonic was actually an android the entire adventure, and the real Tails was being held captive.
From there, Eggman revealed his latest creation: the Egg TMRE-Channeling Champion, in which Sammy was being used as its power source. Eggman tried to escape to June 23rd, 1991 to prevent Sonic from ever defeating him in Final Zone, but Sonic leaped on Eggman's mech, allowing him to follow Eggman through time. Sonic and Eggman raced to Final Zone, traversing through Scrap Brain Zone along the way. At the Final Zone, Sonic dueled Eggman while the 1991 Sonic dueled the 1991 Dr. Robotnik. Both Sonics succeeded, and Sonic freed Sammy from the capsule which he was sealed in. The two traveled back to 2014, freed their imprisoned friends, and went back to Emerald City. Sammy said goodbye to everyone from there and teleported back to Pensacola, Florida. Shortly after, Shelby appeared and congratulated Sammy. She also revealed that she knew Sammy had a crush on her the entire time and that she doesn't have time for romantic relationships. However, she did tell Sammy that he would one day find a sweetheart other than her. She didn't reveal who it would be, leaving Sammy melancholy. However, Timothy and Tiffany cheered him up with a relaxing game of volleyball. The novel ends here.
Sammy Harbors: main protagonist
Sonic the Hedgehog: protagonist
Tails Android: supporting character of main antagonist
Miles "Tails" Prower: supporting character
Knuckles the Echidna: protagonist
Shelby the Multiversal Guardian of Life: supporting character
G.U.N. forces: antagonists (misunderstood)
Shadow the Hedgehog: protagonist
Amy Rose: protagonist
Mighty the Armadillo: protagonist
Ray the Flying Squirrel: minor character
Dr. Eggman: main antagonist