这是一个被动语态，先去掉附加的定语从句we are going to see this evening 为：
The moviewas said to be based on a true event. 这部电影曾被认为基于一次真实事件（ be said to be 表示“被认为是”）
改回到主动语态为：
People said the movie to be based on a true event. （movie 是宾语，不定式to be based on a true event是宾补），意思是“人们曾经认为这个电影立基于一次真实事件的”。
如果用现在时，句子就成了People say the movie to be based on a true event，意思变为“人们现在认为这个电影要立基于一次真实事件”。
本句中的the movie 已经公演，based on a true event已经成为往事，人们对要发生的这件事的传言也应该在过去。如果是现在的传言，说明电影还没有完成，也就不会存在定语从句we are going to see this evening 所说的打算。
其实，这个意思也可以用主语从句 It (is) said that the movie (was) based on a true event 来表示（注意括号中动词的时态！）。
“It is said + that 从句”和“(that从句的)主语 + be said + 不定式”这两种句式可以互换，前者表示“据说某人/某事如何”，后者表示“某人/某事被认为如何”。但是，第二种的be said 一定要和It is said that中从句的时态一致。例如：
It is said that she (is) quite sure of success. (that从句用的是现在时)
改为 She (is) said to be quite sure of success. （She is said也应该用现在时）
It is said she (was) quite sure of success. (that从句用的是过去时)
改为 She (was) said to be quite sure of success. （She was said也应该用过去时）
The case (is) said to be an omen of misfortune. （The case is said 是现在时）
改为 It is said that the case is an omen of misfortune. （that从句也应该用现在时）
The case (was) said to be an omen of misfortune. （The case was said 是过去时）
改为 It is said that the case (was) an omen of misfortune. （that从句也应该用过去时）
