Secret Agent Grandma

 Secret Agent Grandma was the sixteenth book in the Give Yourself Goosebumps gamebook series. It was preceded by Please Don't Feed the Vampire! and followed by Little Comic Shop of Horrors. The cover illustration consisted of a green alien with three eyes peeling away a grandmother disguise in the middle of a flower garden. The tagline was, "Will the real grandma please stay alive!" It was released in April 1997 and was 131 pages long.

Plot
Your parents are going away, so your grandmother is coming to look after you. You haven't seen her since you were a small child, but you're told that she'll be wearing a distinctive purple and yellow outfit. You arrive at the train station and find your grandmother on the platform - but then a train passes and you see an identical woman involved in a fight on the train. You aren't sure whether your grandmother is the one on the platform or the one on the train, and the storyline you follow is determined by which grandma you decide is the right one.

Story A

You get on the train and meet up with your friends Chuck and Ginny, who also happen to be there. The three of you search for your grandmother and discover that she is a secret agent investigating an alien plot.

Story B

You decide that your grandmother is the one on the platform, and go home with her. Your friends Sophie and Andrew come over, and you discover that Grandma is an alien impostor with a plan to take over the world. It's up to you to stop her and find out where your real grandmother might be ...

Side Story C

In story A, you try to ask fellow train passengers for help finding Grandma. Your grandmother then reveals herself to you and explains that she is a secret agent who's been tasked with protecting an important floppy disk.

Bad Endings

 * While you're waiting at the train station, the ground begins shaking, and you run away. But you don't look where you're running, and you run into a garbage truck and land in the smelly trash.
 * You jump onto the train but get caught by a guard. You try to explain that you're looking for your grandmother and you saw her aboard the train. He doesn't believe you, and throws you off the train. The Grandma on the platform (apparently the real one) is so angry that you jumped onto a moving train that she grounds you for a month.
 * Deciding to introduce Andrew and Sophie to your grandma, you come across odd-looking roses in the flower garden. One of them wraps tendrils around you and your friends - and then spits red goo on you, mutating you into a living rose.
 * You and Andrew and Sophie are snooping in your grandmother's bedroom and have found evidence that she is an alien. When you hear her coming, you hide under the bed; but she knows you're there. She turns into her alien form and spits acid all over you and your friends. You melt away from the acid.
 * You, Chuck and Ginny attend an alien meeting, but it turns out to be a convention about voting on galaxy parking rules, real estate taxes, and planetary trade pacts. You end up getting bored to sleep, and an alien security guard throws you out, since your snoring was disturbing the guest speaker.
 * Sophie, Andrew and you ditch Special Agent Bendrey and decide to board a train to find the conductor, only to discover that the passengers turn out to be ghosts. It's implied that you and your friends end up riding the ghost train for all eternity.
 * You, Sophie, and Andrew go to Andrew's house to tell his parents about the alien invasion, but Sophie, Andrew, and his parents are the same type of aliens your grandmother is. Andrew fishes a baby alien out of his pocket, places it on you, and tells it to eat you. It starts to munch on your ear.
 * Just as you, Andrew and Sophie are about to blast spider-like aliens with water guns, one of them tells you that your grandma was keeping them captive, and that they are eternally grateful to you for freeing them. Special Agent Bendrey tells you that they took the alien granny into custody and dashes off before you can even speak to him. You wonder what to do about the alien creatures.
 * You and Grandma go home in a taxi, but you and the driver both become suspicious of Grandma's luggage. You open up her suitcase and a mutant alien rose bush bursts out. One of the roses gobbles up the cab driver, and then your grandmother says that it's allowed to have you as dessert.
 * When the train with the "other" grandmother passed you, you saw a message written on the window in lipstick ("EMPLEH.") You don't know what this means, but when you get home with Grandma, you realize it was "HELP ME" backwards. You try to tell Grandma that someone on the train needs help - but she zaps you with a mind-control ray and forces you to take her to your mother's rose garden. She unearths alien eggs, which all hatch out, ready to take over the world. You're apparently eaten by one of the newborn aliens.
 * You decide to destroy the alien eggs by squishing them. But your foot hits a land mine, which explodes; killing you, Sophie and Andrew and destroying the garden. It turns out that your grandmother planted land mines in the garden for protection.
 * Your grandmother ostensibly defeats the aliens, and is interviewed for the news. But then Grandma transforms into an alien. She informs you that she staged the fight and you're also an alien.
 * You wake up to find that your parents are home and your mother has made scrambled eggs for breakfast. When you've eaten, you realize too late that the eggs were alien eggs which you put in the fridge earlier (implying everyone will die or turn into aliens.)
 * You and your grandmother decide to see who is on the other side of a bathroom door. But a boy rushes into the bathroom and knocks you both to the floor. What's worse, the device Grandma was holding lands in the toilet.
 * You decide to jump off the train to escape alien creatures. But in that moment, you end up going off a bridge, falling all the way to the bottom of a deep chasm.
 * During the meteor shower, Special Agent Bendrey suddenly transforms in front of you, Sophie and Andrew. It turns out he was an alien being who investigated humans. Three more aliens just like Bendrey drag you and your friends inside the train car. Presumably, you end up dead.
 * Having obtained a secret floppy disk, you decide to insert it into your computer. You discover that it contains all the video games from next year, and decide to play them before anyone else. However, you become so wrapped up in the games that you don't notice the thugs in overcoats who have broken in - until it's too late.
 * You turn down the special assignment to save Earth from aliens. But you get scolded at for being a wimp.
 * You have a choice to pick one of two grandmas to see which is the real one. If you pick the one on the left, it is revealed that she is an alien. (Possibly a reference to the book cover)

Good Endings

 * You have a choice to pick one of two grandmas to see which is the real one. If you pick the one on the right, it is revealed that there are three more grandmas. You end up getting spoiled by five grandmas back at your house.
 * Alien invaders take you home but accidentally destroy your mother's rose garden. They dig up the garden, take the alien eggs that "Grandma" planted there, and give you a packet of seedlings to replace your mother's flowers that were destroyed. You plant the seedlings and they quickly sprout into giant, beautiful alien roses.
 * After noticing that some of the baby aliens were destroyed by a radio, you, Andrew and Sophie grab a bunch of music-making devices, killing off a number of the alien spawn. (Possibly a reference to Mars Attack!) When the alien grandma shows up and the devices get smashed, it looks like the three of you are doomed until you realize it was the music that did them in. You and your friends start singing the "Happy Birthday" song, thus finishing off the alien granny and the remaining alien spawn. Afterwards, you think that the three of you should form a singing group.
 * After deciding to pretend to be an alien to the alien thugs, your fingertips suddenly shoot sparks that short-circuit the force field around your grandmother. It's revealed that you and your grandma really are aliens. Together, you make the world yours, put Ginny and Chuck in charge of their very own solar systems, and make the thugs your galactic chauffeurs.