A is for Alien

After Alvin annoys his teacher enough so he can get out of detention early, he sees Brittany go into the school's basement. When he takes a peak, he sees a green light and is convinced an alien is inside. At home he tells Simon who doesn't believe a word of it. The next day Alvin confronts Brittany, who spills nothing, and rushes back to the basement at the end of class only to find a neon-green substance on the floor. Simon analyses it and determines it's silicon-based. Despite Simon's request that he drop the matter, Alvin wakes Simon up and forces him to go back to school where they are caught snooping by a security guard. They run home to find Theodore missing so Simon reveals his secret jet packs. Back at school, they see a UFO which, in reality, is part of a movie set they crash onto landing them in detention.

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S01E04 - Jeanette Enchanted