The movie had a forgettable plot that was hard to remember.

forgettable 是由动词 forget（忘记）加上形容词后缀 -able 构成的形容词，表示"容易被忘记的"或"不值得记住的"。forget 表示"忘记"，-able 表示"能够...的"，所以 forgettable 字面意思是"能够被忘记的"。